IP Biology Upper Sec 09: Infectious Diseases in Humans

These notes align with SEAB GCE O-Level Biology (6093) content used in IP programmes (exams from 2026).

Status: SEAB O-Level Biology 6093 syllabus (exams from 2026) checked 2025-11-30 — scope unchanged; remains the reference for this note.

What you must know

• Infectious diseases caused by pathogens (viruses, bacteria); non-infectious include genetic, lifestyle diseases.
• Virus vs bacteria: viruses are acellular, need host to replicate; bacteria are cells with cytoplasm, membrane, wall, sometimes plasmids.
• Case studies: influenza (droplet spread, fever, body aches), pneumococcal disease (bacterial, can cause pneumonia/meningitis); both preventable by vaccines, hygiene, masks, reducing crowding.
• Immunity: vaccines introduce antigens to stimulate memory cells/antibodies without causing disease.
• Antibiotics kill/inhibit bacteria (not viruses); misuse (overuse, incomplete courses) selects resistant strains.

Detailed notes

• Transmission: droplets, contact, contaminated food/water, vectors (mosquitoes), body fluids. Break chain with hygiene, clean water, cooking, masks, isolation, vector control (remove stagnant water, nets, insecticides).
• Defences: physical/chemical barriers (skin, mucus/cilia, tears, stomach acid); phagocytes engulf; lymphocytes produce antibodies and memory cells—faster secondary response.
• Vaccination: weakened/killed pathogen or antigen stimulates primary response → memory. Herd immunity helps protect those unvaccinated.
• Antibiotics: target bacteria (cell wall/ribosomes); ineffective on viruses. Misuse breeds resistance—complete courses, avoid unnecessary use.
• Distinguish infectious vs non-infectious (genetic, lifestyle) to avoid incorrect examples.

Worked walkthroughs

• Explain how vaccination leads to active immunity via memory cells; contrast with passive immunity (antibodies given, short-lived).
• Describe how vector control reduces dengue/malaria (destroy breeding sites, nets, sprays).
• Outline body response to a cut: clot forms, phagocytes act, lymphocytes produce antibodies if needed.

Pitfalls and fixes

• Saying antibiotics kill viruses—they do not.
• Confusing vaccination with passive immunity.
• Ignoring matching control to transmission route (e.g., water vs droplet vs vector).
• Saying “body becomes immune to antibiotics”—it’s bacteria that develop resistance.

Practice drills

• Give two measures to prevent food- or water-borne disease and explain why they work.