Human Health and Disease (NEET Biology Class 12): Pathogens, Immunity, AIDS & Cancer
Infectious Diseases, Pathogens & Vectors
🎯 NEET priority: High-yield. 4–5 questions a year — about 10% of the Biology paper. Immunity + the disease/pathogen/vector table + cancer make up roughly 80% of this chapter's questions, and cancer (oncogenes, metastasis, carcinogens) has been tested increasingly since 2019.
Infectious diseases — the master table NEET asks from
Disease | Pathogen | Type | Transmission / vector | Key symptoms & facts |
Typhoid | Salmonella typhi | Bacterium | Contaminated FOOD and WATER; enters via the small intestine, migrates through blood | Sustained high fever 39–40°C, weakness, stomach pain, constipation, headache, loss of appetite. Intestinal perforation and death in severe cases. Confirmed by the WIDAL TEST. Classic case: Mary Mallon, 'Typhoid Mary', a cook |
Pneumonia | Streptococcus pneumoniae and Haemophilus influenzae | Bacterium | Inhaling DROPLETS/aerosols from an infected person, or sharing glasses and utensils | Infects the ALVEOLI, which fill with fluid. Fever, chills, cough, headache; in severe cases lips and fingernails turn GREY TO BLUISH |
Common cold | Rhino viruses | Virus | Inhaled droplets from cough/sneeze, or contaminated objects (pens, books, cups, doorknobs, keyboard) | Infects the NOSE and RESPIRATORY PASSAGE but NOT the lungs. Nasal congestion and discharge, sore throat, hoarseness, cough — lasts 3–7 days |
Malaria | Plasmodium (P. vivax, P. malariae, P. falciparum) | Protozoan | Bite of the infected FEMALE ANOPHELES mosquito | MALIGNANT malaria by P. FALCIPARUM is the most serious and can be fatal. Chill and high fever recur every 3–4 days, caused by the toxin HAEMOZOIN |
Amoebiasis (amoebic dysentery) | Entamoeba histolytica | Protozoan | Contaminated food/water; HOUSEFLIES act as MECHANICAL CARRIERS | Lives in the LARGE INTESTINE. Constipation, abdominal pain and cramps, stools with excess mucous and blood clots |
Ascariasis | Ascaris (the common round worm) | Helminth | Contaminated water, vegetables, fruits — eggs excreted in faeces contaminate soil/water | Internal bleeding, muscular pain, fever, anaemia, blockage of the intestinal passage |
Filariasis (elephantiasis) | Wuchereria bancrofti and W. malayi | Helminth | Bite of FEMALE MOSQUITO vectors | Slow chronic inflammation of the LYMPHATIC VESSELS of the lower limbs, over many years. Genital organs often affected, causing gross deformities |
Ringworm | Microsporum, Trichophyton, Epidermophyton | Fungus | From SOIL, or shared towels, clothes, or combs | Dry scaly lesions on skin, nails and scalp with intense itching. HEAT and MOISTURE help them thrive — hence skin folds like the groin and between the toes |
The Plasmodium life cycle — the sequence NEET tests
Plasmodium enters the human body as SPOROZOITES — the infectious form — through the bite of an infected female Anopheles mosquito. The parasites multiply in liver cells, then attack red blood cells, rupturing them and releasing the toxin haemozoin, which causes the chill and high fever recurring every 3–4 days.
When a female Anopheles bites an infected person, parasites enter the mosquito, develop further, and form sporozoites stored in the mosquito's SALIVARY GLANDS — ready to infect the next human. Plasmodium needs TWO hosts — humans and mosquitoes — to complete its life cycle; the female Anopheles is the VECTOR.
Prevention
Personal and public hygiene is critical — especially for food/water-borne diseases (typhoid, amoebiasis, ascariasis). For air-borne diseases (pneumonia, common cold) also avoid close contact with infected persons or their belongings. For vector-borne diseases (malaria, filariasis), the key is eliminating vectors and breeding places — avoid stagnant water, clean coolers regularly, use mosquito nets, and introduce larvivorous fish like Gambusia.
Immunity: Innate, Acquired, Allergy & Lymphoid Organs
Innate immunity — four types of barrier
Barrier | What it is |
Physical | Skin; mucus coating of the epithelium lining respiratory, gastrointestinal and urogenital tracts traps microbes |
Physiological | ACID in the stomach, SALIVA in the mouth, TEARS from the eyes — all prevent microbial growth |
Cellular | Leukocytes — polymorpho-nuclear leukocytes (PMNL/neutrophils), monocytes and NATURAL KILLER lymphocytes in blood, plus MACROPHAGES in tissues — phagocytose and destroy microbes |
Cytokine | Virus-infected cells secrete INTERFERONS, which protect NON-infected cells from further viral infection |
Acquired immunity — pathogen-specific, with memory
First encounter → a low-intensity primary response. A subsequent encounter with the same pathogen → a highly intensified secondary or ANAMNESTIC response, because the body remembers.
B-lymphocytes produce antibodies → humoral immune response (antibody-mediated). T-lymphocytes mediate cell-mediated immunity (CMI); T-cells do NOT secrete antibodies — they help B cells produce them.
An antibody has four peptide chains — two short light and two longer heavy chains — hence represented as H₂L₂. Types include IgA, IgM, IgE, IgG.
Graft rejection is due to cell-mediated immunity — the body distinguishes 'self' from 'non-self'. Tissue and blood group matching are essential before transplant, and the patient still takes immunosuppressants for life.
Allergy & autoimmunity
Allergy is an exaggerated immune response to environmental antigens called allergens (dust mites, pollen, animal dander). The antibodies produced are IgE type. Symptoms — sneezing, watery eyes, running nose, difficulty breathing — come from the release of histamine and serotonin from MAST CELLS. Treated with anti-histamine, adrenalin and steroids.
Autoimmune disease: the body attacks its own self-cells. Rheumatoid arthritis is the NCERT example.
Lymphoid organs
Type | Organs | Role |
PRIMARY lymphoid organs | Bone marrow and THYMUS | Where IMMATURE lymphocytes differentiate into antigen-sensitive lymphocytes. Bone marrow makes all blood cells; the thymus is lobed, near the heart beneath the breastbone, LARGE at birth but shrinks to a very small size by puberty |
SECONDARY lymphoid organs | Spleen, lymph nodes, tonsils, Peyer's patches of the small intestine, appendix | Where MATURE lymphocytes interact with antigen and proliferate into effector cells. The SPLEEN is bean-shaped, contains lymphocytes and phagocytes, and holds a large reservoir of ERYTHROCYTES. LYMPH NODES trap microbes from lymph and tissue fluid |
MALT | Mucosa-Associated Lymphoid Tissue, in the lining of respiratory, digestive and urogenital tracts | Constitutes about 50 PER CENT of the lymphoid tissue in the human body |
AIDS & Cancer
AIDS
AIDS = Acquired Immuno Deficiency Syndrome, caused by HIV — a retrovirus. The widely used diagnostic test is ELISA (enzyme linked immuno-sorbent assay).
Treatment with anti-retroviral drugs is only PARTIALLY effective — it can prolong life but cannot prevent death. Prevention is the best option. In India, NACO (National AIDS Control Organisation) and NGOs lead education efforts.
Preventive steps: making blood-bank blood HIV-safe, ensuring only disposable needles and syringes, free distribution of condoms, controlling drug abuse, advocating safe sex, and regular check-ups in susceptible populations. NCERT's message: "don't die of ignorance" — infected people need help and sympathy, not shunning.
Cancer — increasingly tested since 2019
Normal cells show CONTACT INHIBITION — contact with other cells stops their growth. Cancer cells have LOST this property, so they divide continuously into masses called tumors.
Tumor type | Behaviour |
BENIGN | Remains CONFINED to its original location, does NOT spread, causes little damage |
MALIGNANT | A mass of proliferating NEOPLASTIC/tumor cells that grow rapidly, INVADE and damage surrounding tissue, and STARVE normal cells by competing for nutrients |
METASTASIS — the most feared property of malignant tumors — is when cells sloughed from the tumor travel through the blood, lodge elsewhere, and start a new tumor there.
Cause of cancer (carcinogen) | Detail |
Ionising radiation | X-rays and gamma rays — cause DNA damage leading to neoplastic transformation |
Non-ionising radiation | UV radiation |
Chemical carcinogens | Those in TOBACCO SMOKE are a major cause of LUNG CANCER |
Oncogenic viruses | Cancer-causing viruses carrying VIRAL ONCOGENES |
Cellular oncogenes (c-onc) / proto-oncogenes | Genes ALREADY present in NORMAL cells which, when activated under certain conditions, cause oncogenic transformation |
Detection and diagnosis: BIOPSY — a piece of suspected tissue is cut into thin sections, stained and examined microscopically (histopathological studies) by a pathologist. Blood and bone marrow tests detect increased cell counts in leukemias. Radiography (X-rays), CT (computed tomography) and MRI (magnetic resonance imaging) detect cancers of internal organs.
Why This Matters for NEET
Why this matters for NEET
High-value one-liners: typhoid → Salmonella typhi, confirmed by the Widal test; pneumonia infects the alveoli; common cold infects the nose but NOT the lungs; malaria's infectious form is the sporozoite and the toxin is haemozoin; filariasis affects lymphatic vessels; antibody = H₂L₂; allergy antibody = IgE, from mast cells; graft rejection = CMI; MALT = 50% of lymphoid tissue; AIDS test = ELISA; loss of contact inhibition defines cancer.
Trap: T-cells do NOT secrete antibodies — only B-cells do; T-cells merely help. Also, bone marrow and thymus are PRIMARY lymphoid organs (where lymphocytes mature) while spleen and lymph nodes are SECONDARY (where they meet antigen) — inverting these is a classic error. And proto-oncogenes are NORMAL genes already present in healthy cells — they are not viral in origin, unlike viral oncogenes.
Second trap: malignant malaria is caused by P. falciparum specifically, not by all Plasmodium species. And houseflies in amoebiasis are MECHANICAL carriers (they merely carry the parasite on their body) — unlike the female Anopheles in malaria, which is a true biological vector where the parasite develops.
Test Yourself: MCQs, PYQs & Active Recall
Answer these, then close the article and do an Active Recall. Reveal each answer only after you commit to one.
Practice Questions
Q1. Typhoid fever is confirmed by which diagnostic test?
(a) Widal test
(b) Biopsy
(c) ELISA
(d) Gram staining
Show answer
Answer: (a) — The Widal test confirms typhoid, caused by Salmonella typhi entering via contaminated food and water.
Q2. Pneumonia primarily infects which part of the lungs?
(a) The alveoli
(b) The trachea
(c) The pleura
(d) The bronchi
Show answer
Answer: (a) — Streptococcus pneumoniae and Haemophilus influenzae infect the alveoli, which fill with fluid causing respiratory difficulty.
Q3. In severe pneumonia, a characteristic sign is that the lips and fingernails:
(a) Show no change
(b) Turn grey to bluish
(c) Become white
(d) Turn yellow
Show answer
Answer: (b) — In severe cases of pneumonia, the lips and finger nails may turn grey to bluish in colour.
Q4. The common cold, caused by rhino viruses, infects:
(a) The nose and respiratory passage but NOT the lungs
(b) The lungs
(c) The intestines
(d) The alveoli only
Show answer
Answer: (a) — Rhino viruses infect the nose and respiratory passage but not the lungs; symptoms last 3-7 days.
Q5. The infectious form of Plasmodium that enters the human body is the:
(a) Trophozoite
(b) Gametocyte
(c) Sporozoite
(d) Merozoite
Show answer
Answer: (c) — Plasmodium enters as sporozoites through the bite of an infected female Anopheles mosquito.
Q6. The toxic substance released when Plasmodium ruptures red blood cells is:
(a) Serotonin
(b) Interferon
(c) Haemozoin
(d) Histamine
Show answer
Answer: (c) — Haemozoin, released on rupture of RBCs, is responsible for the chill and high fever recurring every three to four days.
Q7. The most serious form of malaria, which can be fatal, is caused by:
(a) Plasmodium malariae
(b) Plasmodium falciparum
(c) Plasmodium vivax
(d) All equally
Show answer
Answer: (b) — Malignant malaria caused by Plasmodium falciparum is the most serious and can even be fatal.
Q8. In amoebiasis, houseflies act as:
(a) Intermediate hosts
(b) Biological vectors
(c) Mechanical carriers
(d) Definitive hosts
Show answer
Answer: (c) — Houseflies act as mechanical carriers, transmitting Entamoeba histolytica from faeces of infected persons to food.
Q9. Wuchereria bancrofti causes chronic inflammation of the:
(a) Large intestine
(b) Lymphatic vessels of the lower limbs
(c) Alveoli
(d) Liver
Show answer
Answer: (b) — Filarial worms cause slowly developing chronic inflammation of the lymphatic vessels of the lower limbs — elephantiasis.
Q10. Ringworm is caused by fungi belonging to the genera:
(a) Entamoeba and Plasmodium
(b) Ascaris and Wuchereria
(c) Microsporum, Trichophyton and Epidermophyton
(d) Salmonella and Streptococcus
Show answer
Answer: (c) — These three fungal genera cause ringworm, thriving in the heat and moisture of skin folds.
Q11. Interferons, which protect non-infected cells from viral infection, are an example of which innate barrier?
(a) Physical barrier
(b) Cytokine barrier
(c) Physiological barrier
(d) Cellular barrier
Show answer
Answer: (b) — Virus-infected cells secrete interferons — the cytokine barrier of innate immunity.
Q12. The highly intensified response on a second encounter with the same pathogen is called the:
(a) Primary response
(b) Allergic response
(c) Secondary or anamnestic response
(d) Innate response
Show answer
Answer: (c) — Acquired immunity is characterised by memory; the second encounter elicits a highly intensified secondary or anamnestic response.
Q13. An antibody molecule is represented as H2L2 because it has:
(a) Two lipid and two protein chains
(b) Four identical chains
(c) Two heavy chains only
(d) Two heavy and two light peptide chains
Show answer
Answer: (d) — Each antibody has four peptide chains — two short light and two longer heavy chains — hence H2L2.
Q14. Graft rejection following organ transplantation is due to:
(a) Cell-mediated immunity
(b) Innate immunity
(c) Passive immunity
(d) Humoral immunity
Show answer
Answer: (a) — The body distinguishes self from non-self, and cell-mediated immune response is responsible for graft rejection.
Q15. The antibodies produced in an allergic reaction are of which type?
(a) IgA
(b) IgE
(c) IgG
(d) IgM
Show answer
Answer: (b) — Allergens elicit IgE-type antibodies; symptoms result from histamine and serotonin released from mast cells.
Q16. Which of the following is a PRIMARY lymphoid organ?
(a) Thymus
(b) Lymph nodes
(c) Spleen
(d) Tonsils
Show answer
Answer: (a) — Bone marrow and thymus are primary lymphoid organs where immature lymphocytes differentiate; spleen, lymph nodes and tonsils are secondary.
Q17. MALT constitutes approximately what percentage of the lymphoid tissue in the human body?
(a) 50 per cent
(b) 25 per cent
(c) 90 per cent
(d) 10 per cent
Show answer
Answer: (a) — Mucosa-associated lymphoid tissue constitutes about 50 per cent of the lymphoid tissue in the human body.
Q18. The property that normal cells possess but cancer cells lose is:
(a) Cell division
(b) Phagocytosis
(c) Metastasis
(d) Contact inhibition
Show answer
Answer: (d) — Normal cells show contact inhibition — contact with other cells inhibits growth. Cancer cells have lost this property.
Q19. Metastasis refers to:
(a) The formation of benign tumors
(b) Loss of the nucleus
(c) Uncontrolled cell division at the original site
(d) Cells sloughed from a tumor travelling through blood to start new tumors elsewhere
Show answer
Answer: (d) — Metastasis — the most feared property of malignant tumors — is the spread of tumor cells through blood to new sites.
Q20. Genes present in NORMAL cells that can cause oncogenic transformation when activated are called:
(a) Viral oncogenes
(b) Cellular oncogenes or proto-oncogenes
(c) Interferons
(d) Allergens
Show answer
Answer: (b) — Cellular oncogenes (c-onc) or proto-oncogenes exist in normal cells and, when activated, can lead to oncogenic transformation.
NEET Previous Year Questions (PYQs)
Real NEET previous-year questions on this chapter. Immunity, the disease/pathogen table and cancer together make up about 80% of this chapter's questions, so this bank is deliberately larger. Explanations in our own words.
Q21. The vector for malaria is: (NEET PYQ)
(a) Female Anopheles mosquito
(b) Female Culex mosquito
(c) Male Anopheles mosquito
(d) Housefly
Show answer
Answer: (a) — The female Anopheles mosquito is the vector; Plasmodium needs two hosts — humans and mosquitoes — to complete its life cycle.
Q22. Which of the following diseases is caused by a helminth? (NEET PYQ)
(a) Typhoid
(b) Ascariasis
(c) Ringworm
(d) Amoebiasis
Show answer
Answer: (b) — Ascariasis is caused by Ascaris, a helminth (round worm); typhoid is bacterial, amoebiasis protozoan and ringworm fungal.
Q23. Which cells do NOT secrete antibodies? (NEET PYQ)
(a) T-lymphocytes
(b) B-lymphocytes
(c) None of these
(d) Plasma cells
Show answer
Answer: (a) — T-cells themselves do not secrete antibodies — they help B cells produce them. B-lymphocytes produce antibodies.
Q24. The widely used diagnostic test for AIDS is: (NEET PYQ)
(a) ELISA
(b) Biopsy
(c) Widal test
(d) MRI
Show answer
Answer: (a) — Enzyme linked immuno-sorbent assay (ELISA) is the widely used diagnostic test for AIDS.
Q25. Chemical carcinogens present in tobacco smoke are a major cause of: (NEET PYQ)
(a) Liver cancer
(b) Leukemia
(c) Skin cancer
(d) Lung cancer
Show answer
Answer: (d) — The chemical carcinogens in tobacco smoke have been identified as a major cause of lung cancer.
Q26. Cancer detection by examining thin stained sections of suspected tissue under a microscope is called: (NEET PYQ)
(a) Biopsy and histopathological study
(b) ELISA
(c) Radiography
(d) CT scan
Show answer
Answer: (a) — In biopsy, a piece of suspected tissue is cut into thin sections, stained and examined microscopically by a pathologist.
Q27. Rheumatoid arthritis is an example of: (NEET PYQ)
(a) A deficiency disease
(b) An allergic reaction
(c) An infectious disease
(d) An auto-immune disease
Show answer
Answer: (d) — In auto-immune disease the body attacks its own self-cells; rheumatoid arthritis is the NCERT example.
Q28. Which larvivorous fish is introduced into water bodies to control mosquito breeding? (NEET PYQ)
(a) Rohu
(b) Gambusia
(c) Hilsa
(d) Catla
Show answer
Answer: (b) — Introducing larvivorous fish like Gambusia is a recommended measure to control mosquito vectors.
Q29. Allergy symptoms result from the release of histamine and serotonin from: (NEET PYQ)
(a) T-lymphocytes
(b) Mast cells
(c) Neutrophils
(d) Macrophages
Show answer
Answer: (b) — Allergy is due to the release of chemicals like histamine and serotonin from the mast cells.
Q30. The thymus, a primary lymphoid organ, is: (NEET PYQ)
(a) Absent in humans
(b) Large at birth and reduces to a very small size by puberty
(c) Located in the abdomen
(d) Small at birth and enlarges with age
Show answer
Answer: (b) — The thymus is quite large at the time of birth but keeps reducing in size with age, becoming very small by puberty.
Active Recall Prompt
Write everything you can recall about Human Health and Disease, naming each part first: every infectious disease with its pathogen, type, mode of transmission and symptoms (typhoid, pneumonia, common cold, malaria, amoebiasis, ascariasis, filariasis, ringworm); the Plasmodium life cycle; prevention measures; innate immunity (all four barriers); acquired immunity (primary vs secondary response, B vs T lymphocytes, antibody structure, graft rejection); allergy and autoimmunity; the lymphoid organs (primary vs secondary, and MALT); AIDS (pathogen, test, treatment, prevention); and cancer (contact inhibition, benign vs malignant, metastasis, every carcinogen type, and the detection methods). Begin each fact with its topic and end it with a full stop.