Human Health and Disease: Class 12 Biology · Chapter 7

How would you test the claim that a 'hot' temperament causes fevers? Put a thermometer under the person's tongue. In effect, that is how the old theory of humours fell. From Hippocrates in ancient Greece to the Ayurvedic tradition in India, physicians had explained health as a balance of body fluids, or humours. An excess of one humour, for example, was blamed for a 'hot' temperament and for fevers. Thermometer readings showed that people said to be 'hot' in this way had perfectly normal temperatures.

William Harvey's experiments of the 1620s proved that blood circulates in a closed loop, which undermined the old picture of the body further. Ideas reached by reasoning alone gave way to ideas tested by experiment. Modern biology has found a subtler link between mind and body instead. The brain talks to the immune system through nerves and hormones, so stress, sleep and mood really do change how well we resist disease.

Three broad influences shape a person's health:

• genes: disorders a child is born with or inherits from the parents;
• infections by disease-causing organisms;
• lifestyle: the food and water we take in, how much we rest and exercise, and the habits we pick up or avoid.

Being healthy is a broader idea than it sounds. The World Health Organization describes health as complete physical, mental and social well-being, so someone free of disease but lonely or constantly anxious is not fully healthy. Health also pays: healthy people learn and work better, which helps families and the country prosper, they live longer, and fewer babies and mothers die around childbirth.

Staying healthy rests on everyday habits: a balanced diet, personal cleanliness, regular exercise and practices such as yoga. It also rests on what a community does together: vaccinating people, disposing of waste safely, controlling mosquitoes and other carriers of disease, and keeping food and water clean. Knowing how diseases work, the subject of this chapter, is part of the same toolkit.

A disease is a disturbance in the working of the body, of one organ or of several. It reveals itself through signs, which others can observe (a rash, a fever), and symptoms, which only the person feels (pain, tiredness). Diseases fall into two broad groups.

This chapter follows that split. It begins with common infections and the immune system that fights them, then turns to HIV and AIDS and to cancer, and ends with the damage done by drugs, alcohol and tobacco.

7.1 Common Diseases in Humans

A pathogen (रोगजनक) is any organism that causes disease, and pathogens come from almost every branch of life: bacteria, viruses, fungi, protozoans and parasitic worms (helminths). Most of them are parasites that live in or on us. To cause disease a pathogen must get in, survive and multiply, and as its numbers grow it disrupts the work of the tissues it occupies.

Each pathogen has its own answer to the body's hostile conditions. Typhoid bacteria swallowed with food must outlast stomach acid and digestive enzymes. The malaria parasite hides inside liver and blood cells, and ringworm fungi stay in the dead outer layer of the skin, out of reach of most immune cells. The examples below, one or two from each group, show how varied these routes and tricks are, and the last part of the section asks how we can block them.

Bacterial diseases

A fever that climbs to 39–40 °C and refuses to come down for days, with headache, stomach pain, constipation, weakness and little appetite, is the hallmark of typhoid fever. Its cause is the bacterium Salmonella Typhi (written Salmonella typhi in older books). It is swallowed with contaminated food or water, settles in the small intestine and then spreads through the blood to other organs. In severe cases the wall of the intestine can perforate, which may be fatal.

A blood test, the Widal test, looks for the patient's antibodies against the bacterium; growing the bacteria from a blood sample confirms the diagnosis. Some people recover yet go on shedding the bacteria for months or years without feeling ill. If such a healthy carrier prepares food for others, an outbreak can follow, a reminder that someone can spread a disease while looking perfectly well. Antibiotics treat typhoid, and an Indian-made typhoid conjugate vaccine now protects young children.

Breathing in droplets or aerosols from a sick person's cough, or drinking from their cup, can carry the bacteria Streptococcus pneumoniae and Haemophilus influenzae deep into the lungs. There they infect the alveoli, the tiny air sacs where oxygen enters the blood, and the sacs fill with fluid. The result is pneumonia: fever, chills, cough, headache and laboured breathing. When too little oxygen reaches the blood, the lips and fingernails take on a greyish-blue tint, a warning that the illness is serious.

Pneumonia is the biggest infectious killer of young children worldwide, which is why India's immunisation programme now includes vaccines against both these bacteria. Dysentery, plague and diphtheria are other bacterial diseases of humans.

Viral diseases

Colds sweep through classrooms and offices because their viruses travel so easily. Droplets from a cough or sneeze may be breathed in directly, or may land on a phone screen, a lift button or a bus railing, from which hands carry them to the nose. The common cold is caused mostly by rhinoviruses, which infect the nose and upper airway but not the lungs. For three to seven days the person has a blocked or runny nose, sore throat, hoarse voice, cough, headache and tiredness.

Two practical facts follow. Antibiotics act on bacteria, so they do nothing against a cold; taking them anyway only helps antibiotic-resistant bacteria to spread. And because more than a hundred kinds of rhinovirus circulate, getting over one leaves you open to the next, so people catch colds again and again.

Protozoan diseases

Malaria has troubled humans for thousands of years. It is caused by the protozoan Plasmodium; the species P. vivax, P. malariae and P. falciparum each cause a somewhat different illness. Of these, P. falciparum causes the most dangerous form, malignant malaria, which can damage the brain and kidneys and can kill if it is not treated quickly.

The parasite needs two hosts (Fig. 7.1). It enters a person as sporozoites, slender infective cells injected with the saliva of a female Anopheles mosquito. The sporozoites reach the liver through the blood and multiply inside liver cells for one to two weeks without causing symptoms. Thousands of new parasites then burst out and invade red blood cells.

Inside the red blood cells the parasites multiply again, and the infected cells burst in synchronised waves. Each wave releases new parasites along with haemozoin, a dark pigment left over from the haemoglobin they digested, and other parasite debris. These set off the body's fever response: the patient shivers with chills, then burns with fever. With P. vivax the cycle repeats about every 48 hours, with P. malariae about every 72 hours.

Some parasites in the blood develop instead into gametocytes, the sexual stages. A female Anopheles biting an infected person sucks them up with her blood meal. In her gut male and female gametes fuse, and the zygote grows on the gut wall into a cyst that releases thousands of sporozoites. These move to her salivary glands, ready for her next bite. The mosquito is thus both a host, where the sexual stage happens, and the vector (रोगवाहक) that carries the parasite from person to person.

A housefly that walks over faeces and then settles on uncovered food can carry another protozoan, Entamoeba histolytica, on its legs and body. The fly is not infected itself, so it is only a mechanical carrier. Most infections, though, come straight from water or food contaminated with faecal matter. The parasite lives in the large intestine and causes amoebiasis, or amoebic dysentery: abdominal pain and cramps, constipation, and stools with too much mucus and with blood. It leaves the body in the faeces, ready to start the cycle again.

Diseases caused by worms

Helminths are much larger pathogens. The eggs of Ascaris, the common roundworm, pass out in the faeces of an infected person and contaminate soil, water and plants. A child who drinks dirty water or eats unwashed vegetables or fruit can swallow them. The worms grow in the intestine and cause ascariasis; a heavy infection brings internal bleeding, muscle pain, fever, anaemia and even a blocked intestine. To protect children, India holds a National Deworming Day twice a year, when schools and anganwadis give every child a single dose of the deworming drug albendazole.

Mosquitoes, mainly Culex, carry the larvae of filarial worms from person to person. The worms, Wuchereria bancrofti and the related Brugia malayi (once named Wuchereria malayi), then settle in lymphatic vessels, usually of the legs, and live there for years. The vessels stay inflamed, lymph cannot drain, and the limb slowly swells and thickens; the result is lymphatic filariasis, better known as elephantiasis. The genital organs may also be badly affected.

Fungal diseases

Despite its name, ringworm has nothing to do with worms. Fungi of the genera Microsporum, Trichophyton and Epidermophyton grow in the dead outer layer of skin, nails and hair. They thrive wherever skin stays warm and damp, such as the groin and the spaces between the toes. They produce dry, scaly, intensely itchy patches, often ring-shaped with a clearer centre. People pick them up from soil, from pets, or by sharing towels, clothes or combs with someone who is infected.

Preventing infectious diseases

Every pathogen above needs a way into the body, and most measures against infection work by blocking that way in (Fig. 7.2). Good personal hygiene means keeping the body clean, washing hands with soap, and eating and drinking only clean, safe food and water. In public hygiene a community acts together: disposing of waste and sewage safely, cleaning and disinfecting water tanks, reservoirs and pools, and enforcing hygiene wherever food is cooked and served. These steps matter most against typhoid, amoebiasis and ascariasis, whose pathogens travel in what we eat and drink.

Air-borne infections such as pneumonia and the common cold need more. It helps to keep a distance from people who are ill, to cover coughs and sneezes, to air rooms and not to share a sick person's cup or towel. For malaria, filariasis and other vector-borne diseases, the target is the mosquito. Stop it breeding: let no water stand in pots, old tyres and drains, and empty and dry desert coolers every week. Spray insecticide where larvae grow, and stock ponds with fish such as Gambusia that eat the larvae. Stop it biting: sleep under nets and fit wire mesh to doors and windows.

These measures have grown more urgent as dengue and chikungunya spread through Indian towns and cities. Both are viral diseases carried by Aedes mosquitoes, which breed in small pools of clean standing water and, unlike Anopheles, bite mainly during the day.

Biology has also handed us powerful weapons. Vaccination trained so many immune systems that smallpox was wiped out: the World Health Organization declared it eradicated in 1980. Vaccines have since pushed back polio, diphtheria, tetanus, measles and pneumonia, and India was certified polio-free in 2014. Antibiotics and other medicines cure many bacterial and protozoan infections, and biotechnology, which you will study in Chapters 9 and 10, keeps producing newer and safer vaccines.

7.2 Immunity

Count the infections you had last year: perhaps a cold or two. Now think of the millions of microbes you breathed in, swallowed or touched in the same months. Most never got a foothold, because the body defends itself all the time. That protection is called immunity (प्रतिरक्षा): the sum of everything the immune system does to stop pathogens from causing disease. It works in two layers, innate immunity, which we are born with, and acquired immunity, which builds up as we meet particular pathogens.

7.2.1 Innate Immunity

The first line of defence needs no training. Innate immunity is with us from birth and is non-specific: whatever gets in through a cut, streptococcus or staphylococcus, meets the same defences. Its tools fall into four groups (Fig. 7.3).

• Physical barriers block entry. Unbroken skin, with its tough dead outer layer, is the largest. Inside, sticky mucus coats the airways, gut and urogenital tract, and in the airways tiny hair-like cilia sweep trapped microbes up and away.
• Physiological barriers make conditions hostile. Stomach acid kills many swallowed microbes, and tears and saliva contain lysozyme, an enzyme that breaks down bacterial walls.
• Cellular barriers hunt microbes down. Neutrophils, also called polymorphonuclear leucocytes (PMNL), and monocytes in the blood, and macrophages in the tissues, engulf and digest microbes by phagocytosis. Natural killer cells, a kind of lymphocyte, destroy body cells that are infected or abnormal.
• Cytokine barriers pass on warnings. A cell infected by a virus releases proteins called interferons, which make its uninfected neighbours resistant to the virus.

7.2.2 Acquired Immunity

Children who have had measles almost never catch it again, even when an outbreak sweeps their school. That lasting protection is the work of acquired immunity, which has two hallmarks: it is specific, aimed at one pathogen only, and it has memory. The first encounter with a pathogen triggers a primary response, slow to build (a week or two) and modest in size. A later encounter with the same pathogen meets a secondary, or anamnestic, response: memory cells left over from the first round react within days, and far more strongly (Fig. 7.4).

The specialists of acquired immunity are lymphocytes of two kinds, B lymphocytes (B cells) and T lymphocytes (T cells). B cells fight at a distance: they release antibodies (प्रतिरक्षी), proteins that circulate in the blood and lock onto one particular molecule, an antigen (प्रतिजन), on a pathogen or its toxin. T cells release no antibodies. Helper T cells give B cells the signal to start, and other T cells attack infected cells themselves.

Look closely at an antibody and you find a Y built from four polypeptide chains: two identical long heavy chains and two identical, shorter light chains, joined by disulfide (S–S) bonds (Fig. 7.5). Its shorthand, ${\mathrm{H}}_2{\mathrm{L}}_2$, simply counts them. The tips of the two arms differ from one antibody to another and form two identical antigen-binding sites, so one antibody can grip two antigen molecules and clump pathogens together.

The two arms of acquired immunity are named after where they act (Fig. 7.6). Antibodies work in the body fluids, once called 'humours' (recall the opening of this chapter), so the antibody arm is the humoral immune response. The other arm works cell to cell: in cell-mediated immunity (CMI), killer (cytotoxic) T cells recognise body cells that are infected or foreign and destroy them directly. Helper T cells coordinate both arms.

Cell-mediated immunity is the main obstacle in organ transplantation. Every cell surface carries identity molecules that differ from person to person. Put a donor's kidney into a patient and the patient's T cells read those molecules as non-self and attack, so the graft is rejected. Before a transplant, doctors therefore match blood groups and tissue types as closely as possible, and afterwards the patient takes immunosuppressant drugs, usually for life, to restrain the T cells. Even so, a transplant can be the only life-saving option when a heart, liver or kidney fails; an organ from an animal or an unmatched donor would be rejected quickly.

7.2.3 Active and Passive Immunity

Protective antibodies can be home-made or borrowed. Active immunity is home-made: your own B cells produce antibodies after you meet an antigen, whether in a natural infection or in a vaccine. It takes a week or more to build up, but it leaves memory behind and can last for years.

Passive immunity is borrowed: ready-made antibodies are put into the body. It works at once but fades within weeks or months, because the body has not learnt to make those antibodies itself. Nature uses it to shield newborns. During pregnancy a mother's IgG antibodies cross the placenta to the foetus. After birth, colostrum, the thick yellowish milk of the first few days, is rich in IgA that guards the baby's gut. This is one reason early breastfeeding is encouraged.

7.2.4 Vaccination and Immunisation

A vaccine (टीका) is a rehearsal. It presents the immune system with antigens of a pathogen in a form that cannot cause disease. These may be whole microbes that are killed or weakened, harmless pieces such as purified proteins, or inactivated toxins. The body reacts as it would to a real infection, making antibodies and, crucially, memory B and T cells. If the real pathogen arrives later, the memory cells recognise it within days and launch a large secondary response that usually clears it before any symptoms appear. Try different timelines in the lab below.

Sometimes there is no time to wait for an active response. Tetanus toxin acts within days. A person with a deep, dirty wound whose vaccinations are not up to date may therefore be given antitoxin (tetanus immunoglobulin), ready-made antibodies against the toxin, along with a vaccine dose. Snakebite is treated the same way: the antivenom used in Indian hospitals contains antibodies, raised in horses, that neutralise the venom of the country's most common dangerous snakes. Protecting someone with ready-made antibodies like this is called passive immunisation.

Recombinant DNA technology (Chapters 9 and 10) lets scientists insert the gene for one antigen of a pathogen into bacteria or yeast and harvest the protein in bulk. The hepatitis B vaccine is made this way, by yeast cells that carry a gene for a hepatitis B surface protein. Nothing infectious is involved, so such vaccines are safe, and they can be made cheaply in the very large quantities that national programmes need.

7.2.5 Allergies

Have you ever started sneezing as soon as someone swept a dusty room, or found your eyes streaming near flowering grass? Many people react strongly to substances that are harmless to others. An allergy is an exaggerated immune response to such a substance from the environment, which is then called an allergen. House-dust mites, pollen, animal dander (tiny flakes of skin and hair), some foods and insect stings are common allergens.

On first contact with an allergen, a sensitive person's B cells make antibodies of the IgE class. These attach to mast cells, which sit in the skin and in the linings of the nose, lungs and gut (Fig. 7.8). When the allergen returns, it binds the IgE on the mast cells, and the cells release chemicals such as histamine and serotonin. These make small blood vessels leak and airways narrow, causing sneezing, a running nose, watery eyes, itching and, in people with asthma, difficulty in breathing.

To find the culprit, doctors expose the skin to tiny amounts of likely allergens (a skin-prick test) and watch for a small swelling. Antihistamines, steroids and adrenaline (epinephrine) quickly relieve the symptoms; adrenaline is the emergency treatment for anaphylaxis, a rare and severe whole-body reaction. Allergies and asthma are becoming more common among children in Indian cities. Polluted air contributes. Another explanation, the hygiene hypothesis, is still being tested: it suggests that children raised in very clean surroundings, with little early contact with microbes, develop an immune system more prone to over-react.

7.2.6 Auto Immunity

The memory-based immune system that evolved in higher vertebrates depends on telling 'self' apart from 'non-self'. The recognition is so fine that it can pick out a single foreign protein, which is why immunology experiments can use purified molecules as antigens. Occasionally, for genetic and other reasons that are not fully understood, the system turns on the body's own cells. The result is an autoimmune disease, an example of autoimmunity. In rheumatoid arthritis, a common example, immune cells attack the lining of the joints, causing painful swelling; in type 1 diabetes they destroy the insulin-making cells of the pancreas.

7.2.7 Immune System in the Body

The immune system is spread through the whole body. It is made of lymphoid organs and tissues, many kinds of cells, and soluble molecules such as antibodies. What sets it apart is that it can identify a foreign antigen, mount a response against it and keep a record of it for the future. The same abilities lie behind allergies, autoimmune diseases and the rejection of transplants.

Lymphoid organs (लसीकाभ अंग) are where lymphocytes are produced, mature and multiply (Fig. 7.9). The primary lymphoid organs, the bone marrow and the thymus, are where immature lymphocytes become able to recognise particular antigens. Mature lymphocytes then move to the secondary lymphoid organs to meet antigens, multiply and become effector cells. These are the spleen, the lymph nodes, the tonsils, the appendix and the Peyer's patches in the wall of the small intestine.

• Bone marrow makes all the blood cells, lymphocytes included. B cells also mature here, and young T cells leave for the thymus.
• Thymus: a lobed organ beneath the breastbone, near the heart, where T cells mature. It is relatively large in infancy, shrinks after puberty and is largely replaced by fat in adults.
• Spleen: a large, bean-shaped organ packed with lymphocytes and phagocytes. It filters the blood, trapping microbes carried in it, removes worn-out red blood cells and holds a reserve of blood cells.
• Lymph nodes: small solid bodies along the lymph vessels. They filter the lymph draining from the tissues and trap microbes and other antigens in it; the trapped antigens activate the lymphocytes waiting there. Swollen nodes in the neck during a sore throat are a sign of this work.
• MALT (mucosa-associated lymphoid tissue): lymphoid tissue in the linings of the airways, gut and urogenital tract, including the tonsils and Peyer's patches. It makes up about half of all the lymphoid tissue in the body.

7.3 AIDS

AIDS stands for acquired immunodeficiency syndrome. Each word carries meaning. The deficiency is acquired: it develops during a person's life rather than being inherited or present at birth. It is an immunodeficiency, because the immune system can no longer protect the body. And it is a syndrome, a set of symptoms that occur together. Doctors first described AIDS in 1981. Since then more than 40 million people worldwide have died of AIDS-related illnesses; about 40 million live with HIV today, around 25 lakh of them in India.

AIDS is the late stage of infection by the human immunodeficiency virus (HIV). HIV is a retrovirus: its genes are carried as RNA inside a protein core wrapped in a lipid envelope, and once inside a host cell it copies that RNA into DNA.

How HIV spreads

HIV lives in certain body fluids (blood, semen, vaginal fluid and breast milk) and spreads only when one of these carries it into another person's body. That happens in four main ways. Two are sex without a condom with an infected person, and transfusion of infected blood or blood products. The others are the sharing of needles and syringes, as among people who inject drugs, and passage from an infected mother to her baby during pregnancy, birth or breastfeeding. Babies of mothers with untreated HIV, people who need repeated transfusions, people who inject drugs and people with several sexual partners are therefore at higher risk.

Because HIV does not pass by touch or ordinary social contact, there is no reason, medical or moral, to isolate people living with HIV. Shutting them out harms their physical and mental health and makes people afraid to get tested, which helps the virus spread.

Infection and illness are separated by a long gap. Without treatment, typically five to ten years pass between infection and the first signs of AIDS, during which the person may feel quite well yet can still pass the virus on. Only a test can tell.

How HIV weakens immunity

Inside the body, HIV infects macrophages and helper T cells (${\mathrm{T}}_{\mathrm{H}}$ cells), which carry a surface protein, CD4, that the virus uses as a docking site (Fig. 7.10). In the cell, the enzyme reverse transcriptase, which the virus brings with it, copies the viral RNA into DNA. This viral DNA is inserted into the host cell's own DNA, and from then on the cell's machinery reads it and makes new virus particles. Infected macrophages survive and keep releasing virus for a long time, acting as a reservoir.

The new viruses infect more helper T cells, which die in large numbers, and year after year the helper T-cell count falls (Fig. 7.11). Since helper T cells switch on both B cells and killer T cells, the whole of acquired immunity weakens. The person has repeated fevers, diarrhoea and loss of weight. Then opportunistic infections, ones a healthy immune system would easily control, take hold: tuberculosis caused by Mycobacterium, other bacterial, viral and fungal infections, and parasites such as Toxoplasma. In the end the body can no longer defend itself.

Testing, treatment and prevention

HIV infection is diagnosed with blood tests. The ELISA (enzyme-linked immunosorbent assay) detects antibodies against HIV, or viral antigens, in a blood sample, and a positive result is always confirmed with further tests. In India testing is confidential and free at government centres.

There is still no cure, because the viral DNA stays hidden inside long-lived cells. But anti-retroviral therapy (ART), a combination of drugs that block reverse transcriptase and other viral enzymes, stops the virus from multiplying. Taken every day for life, ART lets the helper T-cell count recover, prevents AIDS from developing and gives most people a near-normal lifespan (Fig. 7.11). A person whose treatment keeps the virus undetectable in the blood does not pass HIV on through sex. India has provided ART free of charge since 2004.11Older textbooks describe ART as only partly effective. Modern combination therapy, started early, controls HIV so well that it is now managed as a long-term condition, but it still cannot remove the virus from the body.

No vaccine against HIV exists yet, so stopping new infections depends on prevention. In India the National AIDS Control Organisation (NACO) leads this work with state agencies and many non-governmental organisations, while the World Health Organization and UNAIDS run programmes worldwide. The main measures are:

• screening every unit of donated blood before it is used
• using only new, sterile needles and syringes in hospitals, clinics and anywhere injections are given
• promoting condoms and safer sexual behaviour, and making condoms freely available
• regular testing for people at higher risk, so that anyone infected can start ART early
• testing all pregnant women and treating mothers who have HIV, which almost eliminates transmission to their babies
• helping people who inject drugs with counselling, clean needles and treatment for addiction

Hiding an infection for fear of rejection only helps it spread. People living with HIV need support, not avoidance, and Indian law protects them. The HIV and AIDS (Prevention and Control) Act, 2017, forbids discrimination against them in jobs, schools, housing and health care. Controlling HIV works only when communities, families and health workers act together.

7.4 Cancer

Few words frighten people as much as 'cancer'. Worldwide it causes about one death in six, and India records about 15 lakh new cases a year. Understanding how a normal cell becomes a cancer cell, a change called oncogenic transformation, has driven decades of research, and with it better ways to detect, treat and prevent the disease.

Grow skin cells in a dish and they spread until they cover the bottom in a single layer, then stop: touching their neighbours switches division off. This contact inhibition is one of many controls that keep each tissue the right size and shape. A cancer cell has escaped these controls. It divides when it should not and keeps dividing when crowded, and its descendants pile up into a mass, a tumour (Fig. 7.12).

Tumours come in two kinds. A benign tumour stays put, often wrapped in a capsule, and harms the body mainly if it presses on something important. A malignant tumour is a true cancer. Its neoplastic ('new-growth') cells push into neighbouring tissue, destroy it and compete with it for nutrients. Its most dangerous trick is metastasis: single cells break loose, ride the bloodstream or lymph to a distant organ and found a new tumour wherever they lodge.

Causes of cancer

Agents that can turn normal cells into cancer cells are called carcinogens. They fall into three groups, and most of them work by damaging DNA.

Viruses that cause cancer, oncogenic viruses, carry genes called viral oncogenes. Our own cells contain related genes, proto-oncogenes or cellular oncogenes (c-onc), which normally control cell growth. When a carcinogen mutates them or switches them on at the wrong time, they can drive uncontrolled division. Usually several such changes must build up in one cell before it becomes cancerous, one reason why most cancers appear later in life.

Detecting cancer

Found early, many cancers can be cured, so detection matters. The standard method is a biopsy: a small piece of the suspect tissue is removed, cut into thin sections, stained and examined under a microscope by a pathologist (histopathology). Leukaemias, cancers of the blood-forming cells, show up in blood and bone-marrow tests as abnormal counts of white cells. Other tools include:

• Radiography, X-ray images, useful for bones and lungs.
• CT (computed tomography), which combines many X-ray images taken from different angles into cross-sections and three-dimensional views of internal organs.
• MRI (magnetic resonance imaging), which uses a strong magnetic field and harmless radio waves, with no ionising radiation, to show fine changes in soft tissues.
• Antibodies that bind antigens found only on certain cancer cells, used in blood tests and to locate tumours.
• Molecular tests for inherited gene changes that raise the risk of particular cancers, such as BRCA1 and BRCA2 for breast and ovarian cancer. People who carry them can have more frequent check-ups and avoid the carcinogens they are most sensitive to, such as tobacco smoke for lung cancer.

Treating cancer

Doctors usually combine several treatments, chosen for the type and stage of the cancer. In surgery the tumour is cut out. In radiotherapy, beams of radiation kill tumour cells while sparing the surrounding healthy tissue as far as possible. Drugs that kill dividing cells are used in chemotherapy, some of them specific to particular cancers. Because they also hit normal dividing cells, side effects such as hair loss, anaemia and nausea are common. Finally, immunotherapy helps the patient's immune system, which cancer cells often manage to evade, find and destroy the tumour. An older approach of this kind gives biological response modifiers, for example α-interferon, to rouse the immune system against the tumour.

7.5 Drugs and Alcohol Abuse

Surveys in India and elsewhere show drug and alcohol use rising, most worryingly among young people. Knowing what these substances do to the body, and why they are so hard to give up, helps young people protect themselves and find healthier ways to cope with pressure.

The drugs most commonly abused belong to three groups: opioids, cannabinoids and coca alkaloids. Most come from flowering plants, and a few from fungi.

The opium poppy, Papaver somniferum, is the source of the opioids. Its unripe seed capsules ooze a latex from which morphine is extracted, and acetylating morphine gives diacetylmorphine, sold on the street as heroin or 'smack': a bitter, odourless, white crystalline powder. Opioids act on opioid receptors in the central nervous system and the gut. Heroin is usually snorted or injected. It is a depressant that slows breathing, heart rate and other body functions, which is why an overdose can stop breathing altogether.

The plant Cannabis sativa supplies the cannabinoids. Its inflorescences (flowering tops), leaves and resin, combined in different ways, give marijuana, hashish, charas and ganja. Taken by smoking or eating, cannabinoids act on cannabinoid receptors, found mainly in the brain. They distort perception and memory and affect the heart and circulation, for example by raising the heart rate. Some athletes have misused them, and they are banned in competitive sport.

The most powerful stimulant of the three groups is cocaine, an alkaloid from the leaves of Erythroxylum coca, a shrub native to South America. Usually snorted, it blocks the transport that normally clears the neurotransmitter dopamine away from nerve endings, so dopamine builds up and floods the user with euphoria and energy. Large doses cause hallucinations and can trigger heart attacks. Other plants with hallucinogenic effects include Atropa belladonna and Datura, whose seeds cause many accidental poisonings in India.

Some substances with genuine medical uses are misused too. Morphine is one of the most effective painkillers and sedatives, invaluable after surgery. Barbiturates, benzodiazepines and amphetamines are prescribed for problems such as sleeplessness, anxiety and attention disorders. Plants, fruits and seeds with mind-altering effects have also been part of folk medicine and religious rituals for centuries. Taking such a substance for reasons other than treating an illness, or so much or so often that it starts to damage the way the body or mind works, is drug abuse.

Tobacco

Tobacco is the most widely used addictive substance in India. It is smoked as cigarettes and bidis, chewed as gutkha or khaini or in paan, or taken as snuff, and it reached India about four centuries ago. For many young people it is the first addictive drug they try, and early tobacco use is linked with later use of other drugs.

Tobacco contains thousands of chemicals, among them nicotine, an alkaloid. Nicotine makes the adrenal glands release adrenaline and noradrenaline into the blood, and both hormones raise blood pressure and heart rate. Smoking raises the risk of cancers of the lung, throat and urinary bladder, as well as bronchitis, emphysema, coronary heart disease and gastric ulcers. Chewing tobacco greatly increases the risk of mouth cancer, one of the commonest cancers among Indian men.

Smoke also carries carbon monoxide ($\mathrm{C}\mathrm{O}$). Haemoglobin binds $\mathrm{C}\mathrm{O}$ about 200 times more tightly than oxygen, so in a smoker part of the haemoglobin is always occupied by $\mathrm{C}\mathrm{O}$ and less oxygen reaches the tissues (Fig. 7.13).

Every tobacco pack sold in India carries a large pictorial health warning and a quit-line number, yet tobacco use remains common among both young and old. Because nicotine is so addictive, the surest protection is never to start; people who want to quit do best with counselling and medical support.

7.5.1 Adolescence and Drug/Alcohol Abuse

Adolescence (roughly ages 12 to 18; the World Health Organization uses 10 to 19) is the stretch of life in which a child grows into an adult physically, emotionally and socially. The brain's reward systems mature earlier than the regions that weigh risks, so teenagers are naturally drawn to new experiences. That is healthy, but it also makes adolescence a vulnerable time for mental and emotional development.

Why do some adolescents start using tobacco, alcohol or drugs? The usual pulls are:

• curiosity and the wish for adventure: wanting to find out what it feels like;
• expected benefits: believing that a drink or a drug will relax, energise or help one fit in, and later using it to escape problems;
• stress: pressure to do well in school and competitive examinations;
• image: the idea, spread by films, advertising and social media, that smoking or drinking is 'cool' or grown-up;
• peer pressure: friends who use, and the fear of being left out;
• home life: an unstable or unsupportive family, or adults at home who use these substances.

7.5.2 Addiction and Dependence

People repeat drugs and alcohol because of what they think they gain. What they do not see coming is the addictive power of these substances. Put simply, addiction (व्यसन) is a strong mental craving for the way a substance makes a person feel, the high or a brief escape from worry. The craving keeps them using it long after it has stopped being pleasant and even as it ruins their health.

Repeated use also changes the body. Nerve cells exposed again and again to a drug reduce the number or the sensitivity of the receptors it acts on, a change called tolerance. The same dose now has less effect, so the user takes more, which deepens the change (Fig. 7.14). Even trying a drug once can start this spiral, and without guidance or counselling it is very hard to climb out of it.

Dependence shows itself when the drug is taken away. The body has adjusted to the drug so thoroughly that stopping it suddenly throws it off balance. The result is a withdrawal syndrome (restlessness and anxiety, trembling, nausea and sweating) that disappears if the drug is taken again. That relief is itself a trap. With alcohol and some sedatives withdrawal can bring seizures and endanger life, so stopping should happen under medical care. A dependent person may also abandon social norms to get money for the drug, straining family life and friendships.

7.5.3 Effects of Drug/Alcohol Abuse

In the short term, drug and alcohol abuse shows up as reckless behaviour, vandalism, violence and accidents. An overdose can cause coma and death from failure of breathing or of the heart, or from bleeding in the brain. Mixing drugs, or taking them with alcohol, makes a fatal overdose far more likely. Warning signs that a young person may be misusing drugs or alcohol often show up in four places:

• at school: falling marks and unexplained absences
• at home: changes in sleep, appetite and weight, and neglect of personal hygiene
• in mood: low spirits, tiredness, irritability, or aggressive and rebellious outbursts
• with people: drifting away from family and old friends, frequent quarrels, and losing interest in hobbies

The harm spreads beyond the user. Someone who cannot pay for drugs may turn to stealing, and the whole family can suffer mental and financial distress. People who inject drugs and share needles and syringes risk serious infections such as HIV and hepatitis B and C, which also spread by sexual contact and infected blood. Years of heavy drinking or drug use damage the nervous system and the liver (cirrhosis). Drinking in adolescence raises the chance of heavy drinking in adult life, and alcohol or drugs taken during pregnancy can harm the developing foetus.

Some athletes misuse drugs to boost performance: narcotic painkillers, anabolic steroids that build muscle bulk and strength, diuretics, and hormones. Such doping is banned, and in India the National Anti-Doping Agency (NADA) tests athletes. Anabolic steroids have serious side effects, which can become permanent with long use.

7.5.4 Prevention and Control

Prevention really is better than cure here, because smoking, drinking and drug use usually begin in adolescence. The aim is to spot the situations that push a young person towards these substances and to act early. Parents and teachers carry a special responsibility, and studies show that warm, supportive parenting combined with consistent rules lowers the risk of substance abuse. These measures help:

• Keep expectations realistic. Pressure to excel beyond one's abilities, in marks, sport or anything else, is a major source of the stress that drugs falsely promise to relieve. Each young person's own interests and pace deserve respect.
• Build coping skills. Education and counselling help young people manage stress, accept setbacks as part of life, and find satisfaction in sport, music, reading or yoga.
• Keep talking. Young people who can talk openly with parents or trusted friends get guidance early and have somewhere to put their worry and guilt.
• Notice the signs. Parents, teachers and friends who recognise the warning signs should raise them with a trusted adult; this is help, not betrayal.
• Get expert help. Psychologists, psychiatrists, and de-addiction and rehabilitation centres treat substance use as the health problem it is. With support and persistence, people recover and lead full, healthy lives.