Human Health and Disease

Good health is a state of complete physical, mental and social well-being, not merely the absence of disease. Many human diseases are infectious, caused by pathogens — disease-causing organisms — and knowing each pathogen, its disease and how it spreads is core NEET recall.

Several important diseases are bacterial. Typhoid is caused by Salmonella typhi; it spreads through contaminated food and water, causes sustained high fever, and is confirmed by the Widal test (the historic carrier 'Typhoid Mary' is a famous case). Pneumonia is caused by Streptococcus pneumoniae (and Haemophilus influenzae), affecting the alveoli of the lungs. Common cold is a viral disease caused by rhino viruses, affecting the nose and respiratory passage.

Protozoan diseases are very important. Malaria is caused by Plasmodium (species vivax, falciparum, malariae; P. falciparum is the most serious) and is transmitted by the bite of an infected female Anopheles mosquito (the vector). The parasite multiplies in liver and red blood cells, and the release of the toxin haemozoin causes the typical recurring chills and high fever. Amoebiasis (amoebic dysentery) is caused by Entamoeba histolytica, spread by houseflies and contaminated food/water.

Other groups complete the list. Ringworm is a fungal disease (genera Microsporum, Trichophyton, Epidermophyton) causing scaly skin lesions. Among helminths (worms), Ascariasis is caused by the roundworm Ascaris, and Filariasis (elephantiasis) is caused by Wuchereria (filarial worms), transmitted by mosquitoes, causing gross swelling of limbs. Prevention generally relies on hygiene, clean food and water, vector control (e.g. eliminating mosquito breeding) and vaccination where available. For NEET, memorise the pathogen, type and vector/mode of spread for each disease — especially malaria (Plasmodium, female Anopheles, haemozoin), typhoid (Salmonella, Widal test) and filariasis (Wuchereria).

The body defends itself against pathogens through immunity, of two broad types. Innate immunity is non-specific and present from birth — it responds the same way to any pathogen. It has four kinds of barriers: physical (skin, mucus lining), physiological (acid in the stomach, saliva, tears), cellular (phagocytes such as neutrophils and macrophages, and natural killer cells) and cytokine barriers (interferons released by virus-infected cells).

Acquired (adaptive) immunity is pathogen-specific and develops after exposure. Its hallmark is memory: the first (primary) response to a new pathogen is slow and weak, but the second (secondary) response to the same pathogen is faster and stronger because memory cells 'remember' it. Acquired immunity is carried out by two kinds of lymphocytes: B-lymphocytes produce antibodies that circulate in body fluids — this is humoral or antibody-mediated immunity (AMI) — while T-lymphocytes carry out cell-mediated immunity (CMI) and also help the B-cells.

An antibody (immunoglobulin) is a Y-shaped protein made of four polypeptide chains — two longer heavy chains and two shorter light chains (so it is written H₂L₂). There are five classes: IgG, IgA, IgM, IgE and IgD (for example, IgE is involved in allergies, and IgA is found in colostrum).

Acquired immunity is also classified by how it is gained. Active immunity is when the body makes its own antibodies after exposure to an antigen (by infection or by vaccination); it is slow to develop but long-lasting. Passive immunity is when ready-made antibodies are given directly to the body; it acts immediately but is short-lived — examples are the antibodies a baby receives in colostrum and the anti-tetanus serum given after an injury. Vaccination works on the principle of memory. Related concepts include allergies (over-reaction to harmless antigens, mediated by IgE and histamine) and autoimmunity (the immune system attacking the body's own cells, e.g. rheumatoid arthritis). For NEET, fix the innate vs acquired distinction, B-cell (humoral/AMI) vs T-cell (CMI), antibody structure (H₂L₂, Y-shaped), and active vs passive immunity with examples.

AIDS (Acquired Immuno Deficiency Syndrome) is caused by the Human Immunodeficiency Virus (HIV), a retrovirus (its genetic material is RNA, copied into DNA inside the host). HIV infects and progressively destroys the helper T-lymphocytes (T_H cells) of the immune system. As these cells are depleted, the body's immunity collapses, so the person falls prey to infections (and some cancers) that a healthy immune system would resist.

HIV spreads through (i) unprotected sexual contact with an infected person, (ii) transfusion of infected blood or sharing infected needles, and (iii) from an infected mother to her child (across the placenta or through milk). It does not spread by casual contact like hugging or sharing food. AIDS is commonly diagnosed by the ELISA test. There is no complete cure; anti-retroviral (ART) drugs control the virus and prolong life, and prevention (safe sex, screened blood, clean needles, awareness) is the key control measure.

Cancer is a disease of uncontrolled cell division. Normal cells show contact inhibition — they stop dividing when they touch neighbours — but cancer cells lose contact inhibition and keep dividing, forming a mass called a tumour. A benign tumour stays confined to its site, but a malignant tumour (cancer) grows rapidly, invades surrounding tissue, and its cells can break off and spread through blood or lymph to start new tumours elsewhere — this spread is metastasis, the most dangerous property of cancer.

Cancers are triggered by carcinogens — physical (ionising radiation, UV), chemical (tobacco smoke) and biological (oncogenic viruses) agents — which activate oncogenes (cancer-causing genes derived from normal cellular proto-oncogenes). Cancer is detected by biopsy, blood tests and imaging (CT, MRI), and treated by surgery, radiotherapy, chemotherapy and immunotherapy (e.g. using interferons). For NEET, fix that HIV is a retrovirus attacking helper T-cells, its three modes of spread, ELISA diagnosis, and the cancer essentials — loss of contact inhibition, benign vs malignant, metastasis, carcinogens/oncogenes and treatments.

The misuse of drugs and alcohol is a serious health and social problem, especially among the young. The drugs most commonly abused fall into three groups, each from a particular source. Opioids (such as heroin / 'smack', and morphine) are obtained from the poppy plant Papaver somniferum; they are depressants that slow down body functions. Heroin (diacetylmorphine) is usually taken by injection and is highly addictive.

Cannabinoids (such as marijuana, hashish, charas and ganja) come from the hemp plant Cannabis sativa; they interact with receptors in the brain and affect the cardiovascular system. Cocaine ('coke' or 'crack') is obtained from the coca plant Erythroxylum coca; it is a powerful stimulant of the central nervous system, producing a sense of euphoria followed by dangerous after-effects. Some plants like Atropa belladonna and Datura have hallucinogenic properties.

Repeated use leads to addiction and dependence. Dependence is the tendency of the body to need the drug regularly; stopping it produces unpleasant withdrawal symptoms (anxiety, shakiness, nausea, sweating). With continued use the body also develops tolerance, needing higher doses for the same effect. Besides illegal drugs, tobacco (which contains the stimulant nicotine) and alcohol are widely abused and cause major long-term harm — tobacco is linked to cancers and heart disease, and alcohol damages the liver (cirrhosis) and nervous system.

The harmful effects are wide-ranging: damage to the nervous system and organs, risky behaviour, financial and family breakdown, and, with shared needles, the spread of HIV and hepatitis-B. Prevention focuses on avoiding undue peer pressure, education and counselling, channelling energy into studies/sports, and seeking professional and family help early. For NEET, fix the three drug categories with their source plant and effect (opioids–poppy–depressant; cannabinoids–Cannabis; cocaine–coca–stimulant), the meaning of dependence/withdrawal/tolerance, and tobacco's nicotine.