Nearly a fifth of the NEET Biology paper is drawn from the "Biology and Human Welfare" unit, and most of those questions are pure recall — which mosquito carries which parasite, which cell secretes which antibody, which microbe ferments which product. This is the highest-yield, lowest-effort scoring zone in zoology, provided you organise the facts instead of merely reading them.
What "Health" Means and How Diseases Are Grouped
Health is not simply the absence of illness. It is a state in which the body's organ systems, the mind, and the person's social functioning are all working well together. Balanced nutrition, personal hygiene, regular exercise, clean drinking water, proper disposal of waste, and immunisation programmes all contribute to it; so does the absence of chronic stress. Biological science has improved human health enormously through vaccines, antibiotics, blood transfusion, organ transplantation and diagnostic imaging, which is why average lifespan and infant survival have risen sharply over the last century.
Diseases are broadly of two kinds:
- Infectious (communicable) diseases — caused by living agents (pathogens) that can pass from one host to another, e.g. malaria, typhoid, AIDS, ringworm.
- Non-infectious diseases — not transmitted; arise from genetic defects, lifestyle, degeneration or uncontrolled cell division, e.g. cancer, diabetes, hypertension.
Pathogens are organisms that survive inside a host and damage it. Different pathogens use different portals of entry and different tissues: Ascaris lives in the intestinal lumen, Plasmodium in liver cells and RBCs, Wuchereria in lymphatic vessels. Understanding "site of residence" makes the symptom list logical rather than something to be memorised blindly.
Common Infectious Diseases of Humans
Learn each disease as a four-part packet: pathogen → mode of transmission → organ affected → key symptom.
Bacterial diseases
- Typhoid — Salmonella typhi; enters through contaminated food and water, passes from the small intestine into the blood and spreads to other organs. High fever (39–40 °C), weakness, stomach pain, constipation, headache, loss of appetite. Intestinal perforation is a severe complication. Confirmed by Widal test. Mary Mallon ("Typhoid Mary"), a cook, was a famous healthy carrier who spread it through the food she prepared.
- Pneumonia — Streptococcus pneumoniae and Haemophilus influenzae; infects the alveoli, which fill with fluid, causing severe breathing difficulty. Fever, chills, cough, headache; lips and fingernails may turn greyish-blue in serious cases. Spread by inhaling droplets or by sharing utensils of an infected person.
- Bacteria also cause dysentery, plague and diphtheria.
Viral diseases 4. Common cold — rhinoviruses; infects the nose and respiratory passages but not the lungs. Nasal congestion and discharge, sore throat, hoarseness, cough, tiredness; lasts 3–7 days. Spread by droplets and by contaminated objects such as pens, doorknobs, books.
Protozoan diseases 5. Malaria — Plasmodium (P. vivax, P. falciparum, P. malariae). Malignant malaria caused by P. falciparum can be fatal. Transmitted by the bite of a female Anopheles mosquito. Chill-and-high-fever attacks recur every three to four days. The parasite multiplies in liver cells and then in RBCs, which rupture and release a toxin, haemozoin, responsible for the fever. Sexual reproduction (gametocyte fusion, zygote, oocyst, sporozoite formation) occurs in the mosquito's gut, so the mosquito is the definitive host and humans the intermediate host. Sporozoites in the mosquito's saliva start infection in a new person. 6. Amoebiasis (amoebic dysentery) — Entamoeba histolytica in the large intestine; constipation, abdominal cramps, stools with mucus and blood clots. Houseflies act as mechanical carriers from faeces to food.
Helminth diseases 7. Ascariasis — Ascaris lumbricoides, the intestinal roundworm. Eggs pass out in faeces, contaminate soil, water and vegetables, and are swallowed. Internal bleeding, muscular pain, fever, anaemia and intestinal blockage occur. 8. Filariasis (elephantiasis) — Wuchereria bancrofti and W. malayi; these filarial worms lodge in lymphatic vessels of the lower limbs, causing chronic inflammation and gross swelling; genital organs are often also deformed. Transmitted by female mosquito vectors (Culex).
Fungal diseases 9. Ringworm — Microsporum, Trichophyton, Epidermophyton; dry, scaly, intensely itchy lesions on skin, nails and scalp. Heat and moisture favour growth, so groin and between-the-toes regions are typical sites; spread through soil or shared towels, combs and clothes.
Prevention across the board rests on personal and public hygiene: safe drinking water, proper cooking, disposal of excreta, control of mosquito breeding (removing stagnant water, using larvivorous fish such as Gambusia, spraying, mosquito nets, wire-meshed windows) and vaccination where available.
Immunity: The Body's Defence Machinery
The ability to resist pathogens is immunity, and it comes in two layers.
Innate immunity is non-specific and present from birth. It has four types of barrier:
- Physical — skin; mucus lining of respiratory, digestive and urogenital tracts.
- Physiological — acid of the stomach, saliva, tears.
- Cellular — leucocytes such as neutrophils and monocytes, plus macrophages of tissues, which phagocytose microbes.
- Cytokine — interferons released by virus-infected cells that protect neighbouring cells.
Acquired immunity is pathogen-specific and shows memory: the first encounter gives a weaker primary response, while a later encounter with the same pathogen gives a much faster, stronger secondary response. It is carried out by two lymphocyte populations — B-cells, which secrete antibodies into the blood (humoral or antibody-mediated immunity), and T-cells, which help B-cells and directly attack infected or foreign cells (cell-mediated immunity, the reason for graft rejection).
An antibody molecule is made of four polypeptide chains — two light and two heavy — written as H₂L₂, giving the familiar Y shape. Five classes exist: IgA, IgD, IgE, IgG and IgM.
Acquired immunity is further split by how the antibodies were obtained:
- Active immunity — the host's own immune system makes antibodies after exposure to a live or dead antigen (infection or vaccination). Slow to develop, long-lasting.
- Passive immunity — ready-made antibodies are transferred, e.g. IgA from mother's colostrum to the infant, IgG across the placenta, or antitetanus/antivenom injections. Immediate but temporary.
Vaccination and immunisation exploit memory. A vaccine containing inactivated or weakened pathogens, or their antigenic proteins, generates antibodies plus memory B and T cells; a real infection later is neutralised quickly. Recombinant DNA technology now permits large-scale production of antigens in yeast, as in the hepatitis B vaccine.
Two ways in which the immune system misbehaves:
- Allergy — exaggerated response to harmless environmental antigens (allergens like pollen, mites, dander). Mast cells release histamine and serotonin, producing sneezing, watery eyes, runny nose and difficulty in breathing; treated with antihistamines, adrenaline and steroids.
- Autoimmunity — the body's own molecules are attacked because self-recognition fails, as in rheumatoid arthritis.
The lymphoid organs where lymphocytes originate, mature and interact with antigens are divided into primary (bone marrow and thymus, where lymphocytes mature and become immunocompetent) and secondary (spleen, lymph nodes, tonsils, Peyer's patches of the small intestine, appendix and MALT). The spleen is a large blood filter rich in lymphocytes and phagocytes that also stores erythrocytes; the thymus is large at birth and shrinks with age.
AIDS and Cancer
AIDS (Acquired Immuno-Deficiency Syndrome) is caused by HIV, a retrovirus with an RNA genome inside a protein coat. It spreads through unprotected sexual contact with an infected person, transfusion of infected blood, shared needles among drug users, and from an infected mother to her foetus through the placenta. It does not spread by touch, sharing food or casual contact.
Sequence of events after infection: the virus enters macrophages, its reverse transcriptase copies the RNA into viral DNA, this DNA integrates into the host cell's DNA and directs production of new virus particles. The macrophage becomes a virus factory. HIV then enters helper T-lymphocytes, replicates in them and progressively destroys them. As T-cell numbers fall, the patient becomes prone to opportunistic infections such as Mycobacterium tuberculosis, fungi and Toxoplasma, and suffers weight loss, chronic fever and diarrhoea. There is a long incubation period — months to years. Diagnosis uses the ELISA test; treatment with anti-retroviral drugs prolongs life but does not cure. Prevention centres on safe sex, use of disposable needles, screened blood in blood banks, and awareness campaigns run through NACO and NGOs.
Cancer arises when the normal controls on cell division break down. Healthy cells show contact inhibition — contact with neighbouring cells stops further proliferation — and cancer cells lose it, dividing endlessly to form a mass of cells called a tumour.
- Benign tumours stay confined to their site and cause limited harm.
- Malignant tumours consist of rapidly proliferating cells that invade and damage surrounding tissue, compete for nutrients and starve normal cells, and undergo metastasis — spread via blood or lymph to distant sites where they establish secondary growths, which is what makes cancer difficult to treat and often fatal.
Cancer arises through mutations in genes controlling cell division. Proto-oncogenes are normal genes regulating the cell cycle; when mutated they become oncogenes, driving uncontrolled proliferation. Tumour suppressor genes (such as p53) normally restrain division or trigger death of a damaged cell; loss of their function removes this brake. Physical carcinogens (UV, ionising radiation), chemical carcinogens (tobacco products, industrial chemicals) and oncogenic viruses can all trigger such mutations, so cancer is usually the endpoint of several accumulated genetic hits rather than one event.
Detection uses biopsy and histopathology of tissue, blood and bone-marrow tests for leukaemias, radiography, CT (a 3-D image built from many X-rays) and MRI (detects subtle molecular changes in an affected organ); monoclonal antibodies target tumour-specific molecules for both diagnosis and therapy. Treatment typically combines surgery (removing the tumour mass), radiotherapy (irradiating the tumour while sparing surrounding tissue) and chemotherapy (drugs killing rapidly dividing cells, hence hair loss and nausea, since hair follicles and gut lining are also fast-dividing); immunotherapy using interferons or engineered antibodies to boost the patient's own defences is an increasingly used fourth approach.
Drug and Alcohol Abuse
Adolescence brings excitement alongside anxiety, self-doubt and peer pressure, which is why drug and alcohol use most often begins in the teenage years. "Abuse" means using a substance for a non-medical, usually pleasurable effect, often leading to dependence (craving) and tolerance (needing larger doses for the same effect).
- Opioids — heroin ("smack"), obtained from the latex of the opium poppy Papaver somniferum, is a depressant slowing body function; typically injected, sniffed or smoked, acting on opioid receptors normally targeted by the body's own endorphins.
- Cannabinoids — marijuana, hashish, charas and ganja, from the inflorescence of Cannabis sativa; act on cannabinoid receptors in the brain, affecting the cardiovascular system among other effects.
- Coca alkaloids — cocaine ("crack"), from Erythroxylum coca, blocks the transporter that recycles the neurotransmitter dopamine, causing it to accumulate at the synapse and produce temporary euphoria and strong dependence.
- Tobacco and alcohol are the most widely abused legal substances. Nicotine stimulates adrenal release of adrenaline and noradrenaline, raising blood pressure and heart rate; long-term tobacco use is strongly linked to lung, bladder and throat cancers, bronchitis, emphysema and coronary heart disease.
Signs of abuse in adolescents include falling academic performance, unexplained absence, neglect of personal hygiene, withdrawal, altered sleep and eating habits, and unusual attempts to get money. Common triggers are curiosity, desire for adventure, need for peer acceptance, escape from stress, and unsupportive home environments.
Prevention rests on resisting peer pressure, education about ill effects, early counselling and medical help, and recognising warning signs in friends and family. Some athletes also misuse performance-enhancing substances such as anabolic steroids, banned in competitive sport for both health and fairness reasons.
Common Mistakes and Exam Traps
- Mixing up benign and malignant tumours. Benign tumours stay localised; only malignant tumours invade neighbouring tissue and metastasise to distant sites.
- Confusing the source plants of opioids, cannabinoids and cocaine. Heroin/morphine from Papaver somniferum; marijuana/hashish from Cannabis sativa; cocaine from Erythroxylum coca — NEET frequently swaps these plant-drug pairs.
- Attributing malaria's fever spike to the mosquito bite. The fever is caused by haemozoin released when infected RBCs rupture, not by the bite itself or by liver-stage parasites.
- Reversing active and passive immunity. Active immunity is generated by the person's own system (slow, long-lasting, e.g. vaccination); passive immunity is borrowed ready-made antibodies (fast, short-lived, e.g. antivenom or maternal colostrum). A vaccine is always active immunity, never passive.