Common Diseases, AIDS, Cancer and Drug Abuse

NCERT's Class 12 chapter on Human Health and Disease has five parts: common diseases, immunity, AIDS, cancer, and drug and alcohol abuse. This site's note on Microbes, Immunity and Modern Biotechnology already covers the immunity portion in full: innate versus acquired immunity, B and T lymphocytes, humoral versus cell mediated immunity, active versus passive immunity, and how a vaccine such as the hepatitis B vaccine is built using recombinant DNA. Read that note first if the immune mechanism itself is the gap. This note covers the chapter's other four parts: named pathogens and their diseases, HIV and AIDS as a distinct clinical story, the biology of cancer, and the real physiology behind drug and alcohol abuse.

Health is complete physical, mental and social well-being, not merely the absence of disease, and it is shaped by genetic disorders, infections, and lifestyle together. NCERT splits diseases into infectious (caused by an organism, transmissible person to person) and non-infectious. The organisms behind infectious disease, bacteria, viruses, protozoans, helminths and fungi among them, are called pathogens, and UPSC's favourite question shape here is a simple pairing: which pathogen causes which disease, and by which route.

Common diseases: bacterial and viral infections

Typhoid is caused by the bacterium Salmonella typhi. The pathogen enters the small intestine through food and water contaminated with it, then migrates to other organs through the blood. Symptoms include sustained high fever (39 to 40°C), weakness, stomach pain, constipation, headache and loss of appetite, and severe cases can cause intestinal perforation and death. The Widal test confirms typhoid fever.

Pneumonia is caused by the bacteria Streptococcus pneumoniae and Haemophilus influenzae, which infect the alveoli (air sacs) of the lungs. The infected alveoli fill with fluid, causing serious breathing difficulty; fever, chills, cough and headache are common, and in severe cases the lips and fingernails can turn bluish from oxygen shortage. It usually spreads by inhaling an infected person's cough or sneeze droplets, or even by sharing their glasses and utensils.

The common cold, caused by Rhinovirus, infects the nose and upper respiratory passage but not the lungs, which is why it stays comparatively mild: nasal congestion, sore throat, hoarseness, cough, headache and tiredness typically last three to seven days. It spreads through inhaled droplets and through touching contaminated surfaces such as pens, cups, doorknobs or a shared keyboard.

Viral hepatitis is where UPSC's real trap lives, because the three common named types are easy to blur together but behave very differently. Hepatitis A spreads by the faeco-oral route, through food or water contaminated with faecal matter; it does not cause chronic liver disease and a person usually recovers fully. Hepatitis B spreads through infected blood and body fluids (sexual contact, contaminated transfusions and needles, mother to child transmission); it has had a safe, effective vaccine since the 1980s giving close to complete protection, yet the infection can still turn chronic and progress to cirrhosis and liver cancer if untreated. Hepatitis C also spreads mainly through infected blood (unsafe injections, contaminated transfusions, shared needles); despite decades of research there is still no licensed vaccine against it, roughly seventy per cent of infections turn chronic, and WHO estimated 47 million people living with chronic hepatitis C and about 239,000 deaths a year as of 2024, mostly from cirrhosis and liver cancer. The genuinely good news is treatment: modern direct-acting antivirals now cure more than ninety-five per cent of people treated for hepatitis C, unlike hepatitis B, which can be prevented but not cured once chronic. Keep the asymmetry straight: A is faeco-oral and never chronic; B and C are blood-borne and can both turn chronic; B has a vaccine and C does not.

Common diseases: protozoan, helminthic and fungal infections

Malaria is caused by protozoan parasites of the genus Plasmodium, different species (P. vivax, P. malariae and P. falciparum) causing different severities, with P. falciparum the most serious and potentially fatal form. The parasite needs two hosts, a human and a female Anopheles mosquito, its vector. An infected mosquito injects sporozoites into the bloodstream; these multiply inside liver cells before attacking red blood cells, and it is the rupture of infected red blood cells, releasing a toxic pigment called haemozoin, that produces the recurring chills and high fever, typically every three to four days. Some parasites develop into sexual forms (gametocytes) that a mosquito picks up during a blood meal, completing the cycle in its gut before fresh sporozoites reach its salivary glands to infect the next person.

Amoebiasis (amoebic dysentery) is caused by the protozoan Entamoeba histolytica, which lives in the human large intestine. Symptoms include constipation, abdominal pain and cramps, and stools with excess mucus and blood clots. Houseflies act as mechanical carriers, picking up the parasite from an infected person's faeces and depositing it on food, so water and food contaminated by faecal matter is the usual source.

Ascariasis is caused by Ascaris, the common intestinal roundworm. Symptoms include internal bleeding, muscular pain, fever, anaemia and blockage of the intestinal passage. Its eggs are excreted with an infected person's faeces and contaminate soil, water and produce, and a healthy person picks up the infection through contaminated water, vegetables or fruit.

Elephantiasis (filariasis) is caused by the filarial worms Wuchereria bancrofti and Brugia malayi, which the World Health Organization credits with roughly ninety per cent and most of the remainder of global cases respectively. These worms cause a slow, chronic inflammation, usually of the lymphatic vessels of the lower limbs, that develops over many years and can also badly deform the genital organs. The parasite spreads through the bite of an infected female mosquito, commonly a Culex mosquito, which WHO notes is widespread across urban and semi-urban areas, unlike the rural Anopheles and Pacific-island Aedes vectors that transmit filariasis elsewhere. Keep the three genera apart: Anopheles for malaria, Culex for filariasis, Aedes for the dengue-and-chikungunya cluster common across urban India.

Ringworm is caused by fungi of the genera Microsporum, Trichophyton and Epidermophyton, and is one of the most common infectious diseases in humans. It produces dry, scaly lesions accompanied by intense itching on the skin, nails and scalp. Heat and moisture help these fungi thrive, which is why ringworm is common in skin folds such as the groin or between the toes, and it spreads through contaminated soil, or by sharing towels, clothes or a comb with an infected person.

Prevention across all of these leans on the same basics: personal hygiene (a clean body, clean drinking water, clean food), public hygiene (proper disposal of waste and excreta, disinfecting water reservoirs), and, for the vector-borne diseases specifically, eliminating mosquito breeding sites (removing stagnant water, using mosquito nets and wire mesh on doors and windows, introducing larvae-eating fish such as Gambusia into ponds, and insecticide spraying).

AIDS: from infection to syndrome

AIDS stands for Acquired Immuno Deficiency Syndrome. "Acquired" matters: it is not a disease a person is born with, it develops during a person's lifetime after infection with HIV, the Human Immunodeficiency Virus, a retrovirus whose genome is RNA enclosed in a protein envelope. The virus spreads through contact with infected body fluids: unprotected sexual contact, transfusion of contaminated blood or blood products, sharing needles (a major route among people who inject drugs), and from an infected mother to her child, usually around birth. It is not spread by ordinary day-to-day contact, touch, sharing food or water, or using the same toilet, which is exactly why isolating an infected person achieves nothing and only adds stigma.

HIV specifically targets and progressively destroys helper T lymphocytes, and it is this steady fall in helper T cell numbers, not the initial infection itself, that eventually leaves a person unable to fight off infections a healthy immune system would brush off easily, opportunistic infections caused by organisms such as Mycobacterium, various fungi and protozoans like Toxoplasma. This is the crucial distinction to hold onto for UPSC: HIV infection and AIDS are not the same event. A person can carry HIV for years, usually five to ten, before their helper T cell count falls far enough for AIDS itself, the late, severely immunocompromised stage marked by these opportunistic infections, to set in. A second, much shorter interval is often confused with this one: the "window period" is the gap, typically a few weeks and rarely more than around three months, between infection and the point at which the body has produced enough antibody for a standard blood test to detect it. A person is fully infectious during the window period despite testing negative, which is why a single negative test soon after a risk exposure is not the final word.

ELISA (enzyme-linked immunosorbent assay) is the widely used screening test for HIV, though a single test is never treated as a final diagnosis on its own, a reactive result is always followed by confirmatory testing. There is still no cure for HIV infection, but antiretroviral therapy (ART) has changed what a diagnosis means in practice: it cannot clear the virus from the body, but sustained treatment can suppress it to the point that HIV is now managed as a chronic condition rather than an inevitably fatal one, which is exactly why early testing and prompt treatment matter as much as prevention itself.

Cancer: when growth control fails

Cell growth and division in the body are normally tightly controlled. A normal cell shows a property called contact inhibition, by which contact with neighbouring cells switches off further division. Cancer cells have lost this property, so they keep dividing and pile up into a mass called a tumour. Benign tumours stay confined to their original site and cause comparatively limited damage. Malignant tumours are masses of rapidly proliferating neoplastic (tumour) cells that invade and damage surrounding tissue directly, and starve normal cells of nutrients by competing for them. The single most feared property of a malignant tumour is metastasis: cells shed from the primary tumour travel through the blood, lodge at a distant site, and start a fresh tumour there, turning a localised cancer into a body-wide disease.

Cancer is triggered by agents called carcinogens, grouped as physical (ionising radiation such as X-rays and gamma rays, and non-ionising radiation such as UV), chemical (the carcinogens present in tobacco smoke are a major identified cause of lung cancer), and biological (cancer-causing viruses, which carry genes called viral oncogenes). Normal cells also carry their own dormant cellular oncogenes, or proto-oncogenes, which certain conditions can switch on, triggering the same kind of transformation from within.

Early detection is what makes successful treatment possible. Biopsy, examining a stained thin section of suspected tissue under a microscope (histopathological study), is the definitive diagnostic test, alongside blood and bone marrow cell counts for leukaemias. Imaging carries much of the rest of the load: radiography (X-rays), CT (computed tomography, building a three-dimensional image with X-rays) and MRI (strong magnetic fields plus non-ionising radiation, to pick up pathological change in living tissue) are all in routine use. Antibodies against cancer-specific antigens can flag certain cancers too, and blood tests for named tumour markers add another layer: prostate-specific antigen (PSA), made by both normal and cancerous prostate cells, is one such marker used to monitor prostate cancer and follow up on suspicious symptoms, though a raised PSA level alone does not confirm cancer. Molecular biology can even flag genes that predispose someone to a particular cancer before any tumour appears, letting them actively avoid known triggers, tobacco smoke for someone susceptible to lung cancer, say.

Treatment usually combines surgery, radiotherapy (lethal irradiation of the tumour while shielding surrounding healthy tissue) and chemotherapy (drugs that kill cancerous cells, some tumour-specific, with common side effects such as hair loss and anaemia), and increasingly immunotherapy, where biological response modifiers such as alpha-interferon activate a patient's own immune system to help destroy the tumour, since tumour cells are often able to dodge detection by the immune system on their own. Cancer remains a major global cause of death: the World Health Organization estimated close to ten million cancer deaths worldwide in 2024, roughly one in six deaths overall, with lung, breast, colorectal and prostate cancer among the most common cancers by new cases.

Drug and alcohol abuse

NCERT groups commonly abused drugs into three categories: opioids, cannabinoids and coca alkaloids, most of them sourced from flowering plants.

Opioids bind to specific opioid receptors present in the central nervous system and the gastrointestinal tract. Heroin, street name "smack", is chemically diacetylmorphine, a white, odourless, bitter crystalline compound made by acetylating morphine, which is itself extracted from the latex of the opium poppy, Papaver somniferum. Usually taken by snorting or injection, heroin is a depressant that slows the body down. Morphine itself cuts both ways here: extracted from the same plant, it is also one of the most effective sedatives and painkillers in legitimate medical use, particularly for patients recovering from surgery.

Cannabinoids are chemicals that interact with cannabinoid receptors found mainly in the brain, and are naturally obtained from the inflorescences of Cannabis sativa. Different combinations of the plant's flower tops, leaves and resin produce marijuana, hashish, charas and ganja. They are generally taken by inhalation or oral ingestion and are best known for their effect on the cardiovascular system.

Cocaine, the coca alkaloid, is obtained from the coca plant Erythroxylum coca, native to South America. It interferes with the transport of the neurotransmitter dopamine, and is usually taken by snorting (street names "coke" or "crack"). Cocaine is a potent stimulant, producing a rush of euphoria and energy, and excessive doses can cause hallucinations. Other, otherwise legitimate medicines, barbiturates, amphetamines and benzodiazepines, prescribed for depression or insomnia, are also commonly abused beyond medical supervision, alongside hallucinogenic plants such as Atropa belladonna and Datura. Doping in sport is a related but distinct pattern: narcotic analgesics, anabolic steroids, diuretics and hormones taken to boost strength and aggressiveness carry serious side effects, including masculinisation and abnormal menstrual cycles in females, and reduced testicular size and prostate enlargement in males; in an adolescent of either sex, prolonged use can prematurely close the growth centres of the long bones and stunt growth permanently.

Why adolescence specifically? NCERT frames the twelve-to-eighteen age band as a vulnerable bridge between childhood and adulthood. Curiosity and a natural pull towards adventure, excitement and experimentation are what usually start it; academic pressure to excel in examinations, an image of drug or alcohol use as "cool" reinforced by films and the media, and unstable or unsupportive family circumstances combined with peer pressure are what NCERT names as the factors that push early experimentation towards habitual use, at which point curiosity often turns into an escape from problems.

Repeated use builds tolerance, so a user needs progressively higher doses for the same effect, and the underlying psychological attachment to that effect is addiction. Its physiological counterpart, dependence, shows up as a withdrawal syndrome, anxiety, shakiness, nausea and sweating, sometimes severe enough to need medical supervision, when use is abruptly stopped. Abuse can trigger reckless behaviour and violence, and an overdose, especially a drug combined with alcohol, can cause coma or death from respiratory failure, heart failure or cerebral haemorrhage. Injecting drugs intravenously carries a further risk: sharing needles is a major route for both HIV and hepatitis B, both chronic, potentially fatal infections also spread through sexual contact or infected blood, which links this section directly back to viral hepatitis and AIDS above. Chronic use also damages the nervous system and liver (cirrhosis), and use during pregnancy can harm the developing foetus. Tobacco deserves its own mention: nicotine stimulates the adrenal gland to release adrenaline and noradrenaline, raising blood pressure and heart rate, and smoking is linked to cancers of the lung, urinary bladder and throat, plus bronchitis, emphysema, coronary heart disease and gastric ulcers, while chewing tobacco raises oral cavity cancer risk; smoking also raises blood carbon monoxide, lowering the oxygen the blood can carry.

The exam angle

This chapter rewards precision over recall of a definition. UPSC's recurring trap is a pairing or matching question, so the highest-value preparation is locking in exact pathogen-to-disease and vector-to-disease pairs rather than a general sense of "which disease is which": Salmonella typhi to typhoid, Plasmodium to malaria, Entamoeba histolytica to amoebiasis, Ascaris to ascariasis, Wuchereria to filariasis, and the three ringworm genera together. Vectors are a closely related trap, a question can easily swap Anopheles (malaria), Culex (filariasis) and Aedes (dengue/chikungunya) to test whether a candidate has actually distinguished them or just remembers "mosquito-borne disease" as one blur. The hepatitis A-B-C cluster is a proven favourite for the same reason: three related diseases that differ on transmission route, vaccine availability and chronic risk, easy to state correctly for one type and carelessly transfer to another. On AIDS, hold two different clocks apart, a window period of weeks before a test can detect infection, versus years before AIDS itself sets in, and remember that HIV infection and an AIDS diagnosis are not interchangeable terms. On cancer, benign versus malignant and the specific meaning of metastasis are worth stating precisely. On drug abuse, the plant-to-drug-category pairing (opium poppy to opioids, Cannabis sativa to cannabinoids, coca to cocaine) is the same matching format applied to pharmacology.

Quick revision points

• Bacterial: typhoid (Salmonella typhi, Widal test); pneumonia (Streptococcus pneumoniae, Haemophilus influenzae).
• Viral: common cold (Rhinovirus, nose and respiratory passage, not the lungs); viral hepatitis A (faeco-oral, never chronic), B (blood-borne, vaccine exists, can turn chronic), C (blood-borne, no vaccine, can turn chronic, now curable with direct-acting antivirals in most cases).
• Protozoan: malaria (Plasmodium, Anopheles vector, haemozoin causes the fever cycle); amoebiasis (Entamoeba histolytica, houseflies as mechanical carriers).
• Helminthic: ascariasis (Ascaris); filariasis/elephantiasis (Wuchereria bancrofti and Brugia malayi, commonly a Culex vector).
• Fungal: ringworm (Microsporum, Trichophyton, Epidermophyton).
• AIDS: caused by HIV (a retrovirus), spreads only through body fluids, never casual contact; window period (weeks, before antibodies are detectable) is distinct from the years-long gap before AIDS symptoms appear; ELISA screens, confirmatory testing follows; no cure, but ART now manages it as a chronic condition.
• Cancer: benign tumours stay localised, malignant tumours invade and metastasise; carcinogens are physical, chemical or biological; diagnosis leans on biopsy, radiography, CT, MRI and tumour markers such as PSA; treatment combines surgery, radiotherapy, chemotherapy and immunotherapy.
• Drug abuse: opioids from Papaver somniferum (heroin from morphine), cannabinoids from Cannabis sativa, cocaine from Erythroxylum coca; adolescence (12 to 18 years) is the vulnerable window, driven by curiosity, academic stress, peer pressure and unstable family circumstances; needle-sharing links drug abuse directly to HIV and hepatitis B transmission.

Practise the linked questions below to see exactly how UPSC turns these pathogen pairings and disease distinctions into statement-based traps.