Human Health and Disease — NEET Notes

What "health" actually means

The chapter opens with a definition the examiner returns to year after year. Health is not the mere absence of disease, nor is it simple physical fitness. The World Health Organization formulation, echoed verbatim by NCERT, is broader: a state of complete physical, mental and social well-being. The Greek and Ayurvedic notion of balanced "humors" — discredited the moment William Harvey demonstrated blood circulation and thermometers showed normal body temperatures in supposedly "blackbile" patients — has been replaced by a layered understanding in which the neural and endocrine systems modulate immunity, and immunity in turn keeps the organism intact. Three categories of factors threaten that intactness: genetic disorders inherited at birth, infections acquired during life, and lifestyle (food, water, exercise, rest, habits).

"Health is a state of complete physical, mental and social well-being — not merely the absence of disease or infirmity."

The WHO definition — NCERT echoes it directly

Diseases divide into two camps. Infectious diseases are caused by pathogens — bacteria, viruses, fungi, protozoans, helminths — that transmit from person to person, often through air, water, food, body fluids or vectors. Non-infectious diseases do not transmit; they arise from genetic, lifestyle or degenerative causes. Cancer is the leading non-infectious killer. The 2026 chapter examined here keeps to NCERT scope and excludes the rationalised diabetes/hypertension subtopics now read from NIOS.

Bacterial diseases — typhoid and pneumonia

Two bacterial infections anchor the NCERT list. Typhoid is caused by Salmonella typhi, a Gram-negative bacterium that enters the small intestine through contaminated food or water and then disseminates through the bloodstream to other organs. Sustained high fever (39° to 40°C), weakness, stomach pain, constipation, headache and loss of appetite are the diagnostic cluster. In severe untreated cases, intestinal perforation and death may occur. The Widal test is the standard serological confirmation — every NEET aspirant should pair "Salmonella typhi" with "Widal" automatically. NCERT also flags the historical case of Mary Mallon, "Typhoid Mary," the New York cook who carried S. typhi asymptomatically and spread it through the food she prepared for years.

Pneumonia is caused chiefly by Streptococcus pneumoniae, with Haemophilus influenzae as a frequent secondary culprit. These bacteria infect the alveoli — the air-filled sacs of the lungs — which then fill with fluid, producing respiratory distress. Fever, chills, cough and headache are the early symptoms; in severe disease, the lips and finger-nails may turn gray to bluish, a marker of falling blood-oxygen saturation. Transmission is droplet-borne: inhaling aerosols from an infected person or sharing utensils with them.

Viral diseases — the common cold

Of the viral diseases listed in NCERT, the common cold is the lead example — caused by rhinoviruses, a genus with over 100 serotypes that target the nose and respiratory passage but not the lungs. Nasal congestion and discharge, sore throat, hoarseness, cough, headache and fatigue last three to seven days. The transmission route is the same as pneumonia — droplets from coughs and sneezes, plus indirect spread through pens, books, cups, doorknobs and keyboards. The NEET examiner rarely asks symptoms here; the test is almost always "agent of common cold = rhinovirus."

Protozoan diseases — malaria and amoebiasis

Two protozoan diseases sit at the centre of the chapter, and malaria is by far the more heavily tested.

Malaria is caused by Plasmodium, with four species producing different forms — P. vivax, P. malariae, P. ovale and P. falciparum (the most virulent, responsible for malignant malaria). The vector is the female Anopheles mosquito, which transmits the infectious stage — sporozoites — through its bite. Sporozoites first multiply within the liver cells, then attack and rupture red blood cells. The RBC rupture releases haemozoin, a toxic pigment responsible for the periodic chill and high fever recurring every three to four days. When a female Anopheles bites an infected person, the parasites enter the mosquito's body, multiply, form sporozoites in its salivary glands, and the cycle restarts at the next bite. Two hosts are obligatory — human and mosquito — and the female Anopheles is both vector and definitive host.

Amoebiasis (amoebic dysentery) is caused by Entamoeba histolytica, a protozoan parasite of the large intestine. Symptoms are constipation, abdominal pain and cramps, and stools loaded with excess mucus and blood clots. Houseflies act as mechanical carriers — they ferry the parasite from faeces of an infected person to food. The primary vehicles of infection are faecally contaminated drinking water and food.

Helminth and fungal diseases

Two helminth (worm) diseases appear in NCERT, plus one fungal disease that is easy to misclassify.

Ascariasis is caused by Ascaris lumbricoides, the common round-worm, an intestinal parasite. Symptoms include internal bleeding, muscular pain, fever, anaemia and blockage of the intestinal passage. Eggs are excreted in faeces of infected persons, contaminate soil, water, plants and vegetables, and a healthy person acquires the infection by ingesting contaminated material.

Elephantiasis (filariasis) is caused by Wuchereria — W. bancrofti and W. malayi — the filarial worms. These pathogens produce a slowly developing chronic inflammation of the lymphatic vessels, usually of the lower limbs, and the resulting gross swelling and deformity gives the disease its name. The genital organs are also commonly affected. Transmission is by the bite of female mosquito vectors.

Ringworm is the classic fungal disease in this chapter — and the most-trapped item on the NEET paper. Despite the name, no worm is involved. Members of the genera Microsporum, Trichophyton and Epidermophyton produce dry, scaly lesions on skin, nails and scalp, accompanied by intense itching. Heat and moisture favour fungal growth, which is why ringworm thrives in skin folds — groin, between toes. It is acquired from contaminated soil, towels, clothes or even shared combs.

Innate immunity — four barriers, present at birth

Immunity is the overall ability of the host to resist disease-causing organisms — and NCERT splits it into two arms: innate and acquired. Innate immunity is the non-specific defence we are born with. It does not need prior exposure, it does not discriminate between pathogens, and it acts immediately. NCERT enumerates exactly four kinds of barriers, and the NEET examiner expects all four named in order.

Acquired immunity — specific, memory-bearing

Acquired immunity is pathogen-specific and bears memory. The first encounter with a pathogen produces a slow, low-intensity primary response. A subsequent encounter with the same pathogen elicits a rapid, high-intensity secondary (anamnestic) response. That intensification is the whole basis for vaccination. Two cell types do the work: B-lymphocytes and T-lymphocytes. B-cells secrete antibodies — Y-shaped proteins of structure H₂L₂ (two heavy + two light chains), in five classes (IgA, IgM, IgE, IgG, IgD). Because antibodies circulate in blood and lymph, this arm is called the humoral immune response. T-cells do not secrete antibodies; they themselves recognise and destroy infected cells, producing the cell-mediated immune response (CMI). CMI is what rejects transplanted kidneys, hearts and livers — which is why tissue and blood-group matching, plus lifelong immunosuppressants, are required before any graft.

Active vs passive immunity

Acquired immunity comes in two flavours that the NEET examiner has confused candidates with for at least a decade.

Active immunity arises when the host's own immune system produces antibodies after being exposed to an antigen — either through natural infection or through deliberate immunisation (vaccination). It is slow to develop and takes time to give its full effective response, but it is long-lasting and bears memory. Passive immunity, by contrast, is the direct transfer of ready-made antibodies to the host. It acts quickly but is short-lived and leaves no memory. The yellowish colostrum secreted in the first days of lactation, rich in IgA, gives the newborn naturally acquired passive immunity. A foetus also receives some antibodies across the placenta from the mother. Tetanus antitoxin and anti-snake-venom serum (antivenom) are artificial passive immunisations — pre-formed antibodies injected to neutralise a toxin before the host's own response can develop.

Vaccination and immunisation

The principle of vaccination exploits immune memory. A preparation of antigenic proteins from a pathogen — or the inactivated/weakened pathogen itself — is introduced into the body. The host's immune system mounts a primary response, generating both antibodies and a pool of memory B and T cells. On subsequent natural exposure to the actual pathogen, these memory cells recognise it instantly and unleash a massive secondary response — overwhelming the invader before symptoms develop. Smallpox has been completely eradicated by this approach; polio, diphtheria, pneumonia and tetanus are substantially controlled. Recombinant DNA technology now produces antigenic polypeptides in bacteria or yeast on a large scale — the hepatitis B vaccine grown in yeast is the standard NCERT example.

In some emergencies, a primary response is too slow. For a patient with active tetanus, or one who has just been bitten by a venomous snake, pre-formed antibodies must be injected directly. This is passive immunisation — and it is why antitoxin and antivenom are the same idea, dressed in different vials.

Allergies — when the immune system overreacts

An allergy is an exaggerated immune response to substances in the environment that are otherwise harmless. The substances are called allergens — common ones are dust mites, pollen, animal dander and certain foods. The antibody class responsible is IgE. On allergen contact, IgE-armed mast cells release histamine and serotonin, producing the familiar cascade: sneezing, watery eyes, running nose, difficulty breathing. Anti-histamines, adrenalin and steroids reduce the symptoms. NCERT notes that modern protected, hyper-clean lifestyles may be lowering immunity and increasing allergen sensitivity, especially in metro children — an instance of the hygiene hypothesis.

Autoimmunity — when the immune system attacks self

Memory-based acquired immunity in higher vertebrates depends on the ability to discriminate self from non-self. When that discrimination fails — for genetic or unknown reasons — the body attacks its own cells. This is autoimmunity. Rheumatoid arthritis is the NCERT-flagged example: the immune system attacks the synovial membrane lining the joints, causing inflammation, deformity and pain. Other autoimmune diseases the NEET examiner has tested include psoriasis, vitiligo and myasthenia gravis (a chronic autoimmune disorder of the neuromuscular junction causing fatigue, weakening and paralysis of skeletal muscle). NEET has rejected Alzheimer's disease, muscular dystrophy (genetic) and tetany (Ca²⁺-deficiency) as autoimmune.

The immune system in the body — lymphoid organs

The human immune system is a distributed apparatus: lymphoid organs, tissues, cells and soluble molecules like antibodies. The organs split cleanly into two groups, and NEET tests the division directly.

Primary lymphoid organs are where immature lymphocytes are produced and matured into antigen-sensitive cells. These are exactly two — the bone marrow and the thymus. Bone marrow is the master factory for all blood cells, including all lymphocytes. The thymus is a lobed organ near the heart, beneath the breastbone; it is large at birth but shrinks dramatically by puberty. Both organs provide microenvironments where T-lymphocytes mature. After maturation, lymphocytes migrate to secondary lymphoid organs — spleen, lymph nodes, tonsils, Peyer's patches of the small intestine, and the appendix. The spleen filters blood-borne microbes and stores erythrocytes; the lymph nodes trap microbes from lymph and tissue fluid. Mucosa-associated lymphoid tissue (MALT) lines the respiratory, digestive and urogenital tracts and constitutes about 50 percent of the lymphoid tissue in the human body — a number NEET 2017 examined verbatim.

AIDS — the retrovirus that targets immunity itself

AIDS stands for Acquired Immuno Deficiency Syndrome — a deficiency of the immune system acquired during life (not congenital), recognised in 1981 and responsible for more than 25 million deaths since. The causative agent is the Human Immunodeficiency Virus (HIV), a member of the retrovirus family — enveloped RNA viruses that reverse-transcribe their genome into DNA inside the host.

HIV spreads through four routes only: sexual contact with an infected person, transfusion of contaminated blood, sharing of infected needles (intravenous drug abuse), and from infected mother to child across the placenta. Casual touch does not spread HIV — a fact NCERT emphasises to argue against the stigmatisation of patients. Between infection and the appearance of AIDS symptoms there is a lag of months to many years, typically 5–10.

Inside the body, the virus enters macrophages, where its RNA is reverse-transcribed by the viral enzyme reverse transcriptase into DNA. This viral DNA integrates into the host cell's chromosome and directs production of new virus particles. Macrophages become a sustained virus factory. Simultaneously, HIV enters helper T-lymphocytes (T_H cells, CD4⁺), replicates, and produces progeny viruses that attack still more T_H cells. The number of helper T-cells falls progressively. As immune competence collapses, the patient suffers opportunistic infections from Mycobacterium, fungi, parasites like Toxoplasma — pathogens a healthy immune system would shrug off. The standard diagnostic is the ELISA test (enzyme-linked immunosorbent assay). Anti-retroviral drugs prolong life but do not cure; death is, in NCERT's word, "inevitable."

Cancer — when growth control fails

Normal cells show contact inhibition: contact with neighbouring cells stops uncontrolled division. Cancer cells have lost this property. They continue to divide, producing masses called tumours. NCERT separates these into two kinds, and the distinction is on every NEET paper that asks about cancer.

Cancers are classified by tissue of origin. Carcinomas arise from epithelial tissue (breast, lung, colon, skin). Sarcomas arise from connective tissue (bone, muscle, cartilage). Leukaemias are cancers of blood-forming cells in the bone marrow. Lymphomas are cancers of the lymphoid tissue (e.g., Hodgkin's, non-Hodgkin's).

The agents that turn normal cells cancerous are called carcinogens. Ionising radiations (X-rays, gamma rays) and non-ionising radiations (UV) damage DNA and trigger neoplastic transformation. Chemical carcinogens in tobacco smoke are the leading cause of lung cancer. Cancer-causing viruses — oncogenic viruses — carry viral oncogenes; the host genome itself contains cellular oncogenes (c-onc) or proto-oncogenes that, when activated by mutation, drive oncogenic transformation. NEET 2016 (Q.81) tested the corollary: mutations increase telomerase production in cancer (they do not inhibit it), since unchecked telomerase activity is part of what makes cancer cells effectively immortal.

Detection rests on biopsy (a stained tissue section examined microscopically), blood and bone-marrow counts (for leukaemias), and imaging — radiography, CT, MRI. Antibodies against cancer-specific antigens are used for some cancers, and molecular techniques can identify inherited susceptibility genes. Treatment combines surgery, radiotherapy (lethal irradiation of tumour cells), and chemotherapy. Immunotherapy uses biological response modifiers like α-interferon to activate the patient's immune system against the tumour.

Drugs and alcohol abuse

The three classes of commonly abused drugs in NCERT are opioids, cannabinoids and coca alkaloids — most plant-derived, a few fungal in origin.

Opioids bind to opioid receptors in the central nervous system and gastrointestinal tract. Heroin (smack) is diacetylmorphine — a white, odourless, bitter crystalline compound — produced by acetylation of morphine, which is extracted from the latex of the opium poppy Papaver somniferum. Heroin is a depressant that slows down body functions. Morphine itself is an effective sedative and painkiller, of immense clinical value after surgery.

Cannabinoids interact with cannabinoid receptors principally in the brain. Natural cannabinoids come from inflorescences of Cannabis sativa. Flower tops, leaves and resin in various combinations yield marijuana, hashish, charas and ganja. NCERT specifically flags cannabinoids' effects on the cardiovascular system.

Coca alkaloid (cocaine, "coke" or "crack") is extracted from Erythroxylum coca, native to South America. Its mechanism is precise and NEET-favourite: it interferes with the transport of the neurotransmitter dopamine. The result is a potent CNS stimulant — euphoria and energy in low doses, hallucinations in excess. Atropa belladonna and Datura are other plants with hallucinogenic properties.

Tobacco, smoked or chewed, contains nicotine — an alkaloid that stimulates the adrenal gland to release adrenaline and noradrenaline, raising blood pressure and heart rate. Smoking is associated with cancer of the lung, urinary bladder and throat, plus bronchitis, emphysema, coronary heart disease and gastric ulcer. Smoking also raises blood carbon monoxide, reducing haemoglobin-bound oxygen and producing systemic oxygen deficiency.

Two concepts complete the section. Addiction is a psychological attachment to the perceived benefits (euphoria, temporary well-being) that drives continued use even when self-destructive. With repetition, receptors develop tolerance — higher doses are needed for the same effect. Dependence is the tendency to manifest a characteristic withdrawal syndrome (anxiety, shakiness, nausea, sweating) if regular drug or alcohol intake is abruptly stopped. NCERT prescribes parental and educator vigilance for the danger signs — dropping grades, hygiene neglect, isolation, mood swings — and recommends counselling, healthy diversions, and professional rehabilitation.