How this chapter appears in NEET:

• Consistently contributes 3–4 questions per year, making it one of the top Biology chapters by yield.
• Questions are almost entirely direct factual recall — not analytical. Knowing the exact pathogen, vector, diagnostic test, or antibody class is sufficient.

Highest-frequency question clusters:

1. HIV/AIDS — "Which cells does HIV destroy?" (CD4+ T-helper) tested in 2019, 2022, 2023. Always watch for distractors: B-cells, cytotoxic T-cells, NK cells.
2. Mosquito–disease vector mapping — "Female Anopheles → malaria" tested in 2019, 2021. The Culex–filariasis and Aedes–dengue associations appear as distractors every year.
3. Widal test — linked to typhoid (2018, 2020). Distractors: ELISA (HIV), blood smear (malaria).
4. Active vs passive immunity — one question nearly every year on onset, duration, memory cells, or source of antibody.
5. Immunoglobulin specifics — IgA in colostrum vs IgG crossing placenta; IgE in allergy.

Common trap types:

• Swapping Anopheles and Culex mosquitoes.
• Confusing ELISA (screening) with Western blot (confirmation) for HIV.
• Assuming passive immunity provides long-lasting protection (it does NOT).
• Believing cocaine is a depressant (it is a stimulant).

Revision priority order for NEET 2026:

1. HIV–CD4+ T-cell–reverse transcriptase–ELISA
2. Malaria (Anopheles) vs Filariasis (Culex)
3. Active vs Passive Immunity table
4. IgA/IgG/IgE antibody roles
5. Drug sources (Papaver, Erythroxylum, Cannabis)