Cambridge O Level Biology 5090: Disease and immunity (5090) – Revision Notes

For full explanations and worked examples, use the study guide for this topic. These notes condense Topic 12, Disease and immunity, of the Cambridge O Level Biology 5090 syllabus for examination in 2026, 2027 and 2028: every learning outcome in sections 12.1 Disease, 12.2 Antibiotics and 12.3 Immunity. The syllabus is not tiered, so all of it applies to every candidate.

When you are ready, move on to the practice questions. The course hub and the printable checklist cover the rest of the course, and the free 10-minute diagnostics show where to focus. Airway mucus and cilia are revised in Human gas exchange.

Key definitions

Term	Definition to learn
Pathogen	A disease-causing organism
Transmissible disease	A disease in which the pathogen can be passed from one host to another
Vector	An organism that carries a pathogen between hosts (e.g. the mosquito)
Parasite	An organism living in or on a host and harming it (e.g. the malarial pathogen)
Drug	Any substance taken into the body that modifies or affects chemical reactions in the body
Active immunity	Defence against a pathogen by antibody production in the body
Passive immunity	Short-term defence against a pathogen by antibodies acquired from another individual
Antigen	Molecule on a pathogen with a specific shape; each pathogen has its own
Antibody	Protein that binds to a specific antigen; complementary in shape

12.1 Disease

Transmission (12.1.3)

• Direct contact: blood or other body fluids.
• Indirect: contaminated surfaces or food, animals, the air.

Barriers (12.1.4) – only these four

Barrier	Action
Skin	Tough barrier; pathogens enter through cuts
Nose hairs	Trap particles carrying pathogens
Mucus	Traps pathogens in airways
Stomach acid	Kills pathogens in food and drink

Malaria (12.1.5–12.1.7)

• Pathogen: Plasmodium, a single-celled parasite. Vector: female mosquito.
• Mosquito bites infected person → parasite in mosquito → bite passes it into a new person’s blood → liver → red blood cells.

Life-cycle stage	Control
Eggs, larvae, pupae in still water	Drain or cover still water
Larvae and pupae breathe at the surface	Oil film on the water
Larvae	Larva-eating fish; larva-killing bacteria
Adults	Insecticide spraying; treated bed nets; repellent

HIV (12.1.8–12.1.11)

• HIV is a viral pathogen; infection may lead to AIDS.
• Transmitted by: unprotected sex, shared needles, infected blood transfusion, mother to baby (pregnancy, birth, breast milk).
• Controlled by: condoms, sterile needles / no sharing, screening blood, testing, antiviral drugs, education. No vaccine.

Cholera (12.1.12–12.1.14)

• Caused by a bacterium; transmitted in contaminated water.
• Controlled by: clean water supply, hygienic food preparation, personal hygiene, waste disposal, sewage treatment (stages not needed).

Method in steps: the cholera chain

toxin from bacterium
→ chloride ions secreted into small intestine
→ water potential of gut contents falls
→ water moves into the gut by osmosis
→ diarrhoea, dehydration, loss of ions from blood

Alcohol (12.1.15)

Depressant · reduced self-control · slower reaction times · liver damage · social implications (accidents, violence, family and work problems).

Tobacco smoke (12.1.16)

Component	Effect
Nicotine	Addictive; raises heart rate and blood pressure
Tar	Carcinogens; stops cilia working, mucus builds up
Carbon monoxide	Combines with haemoglobin; less oxygen carried

Strong association with: bronchitis, emphysema (alveolar walls break down, less surface area), lung cancer, heart disease, and reduced birth weight when the mother smokes in pregnancy.

12.2 Antibiotics

• Antibiotics treat bacterial infections. They kill bacteria but do not affect viruses.
• Resistance (e.g. MRSA): resistant bacteria survive an antibiotic and reproduce; non-resistant ones die.
• Minimise it by using antibiotics only when essential.

12.3 Immunity

Antibodies at work (12.3.2–12.3.4)

1. Each pathogen has antigens with a specific shape.
2. Lymphocytes make antibodies with a complementary shape.
3. Antibodies bind to the antigens.
4. This directly destroys the pathogen or marks it for phagocytes to destroy.

Vaccination (12.3.5–12.3.7)

Method in steps

1. Weakened pathogens or their antigens are given.
2. Antigens stimulate lymphocytes to produce antibodies.
3. Memory cells are produced → long-term immunity.

Why vaccination controls spread: vaccinated people do not become ill or pass the pathogen on; with most of a population immune, the pathogen cannot find new hosts.

Passive immunity (12.3.8–12.3.10)

• Routes (only these): across the placenta, in breast milk.
• Breast-feeding gives a newborn antibodies while its own immune system is still developing.
• No memory cells → short-term.

HIV and immunity (12.3.11)

Fewer lymphocytes → reduced antibody production → weakened immune system.

Worked reminders

Cholera in three marks. Explain how the cholera bacterium causes dehydration. The toxin causes chloride ions to be secreted into the small intestine. Water moves into the gut by osmosis. Water and ions are lost from the blood in diarrhoea, so the body is dehydrated. Three links, three points: do not stop at “the toxin causes diarrhoea”.

Smoking in two marks. Explain why a smoker becomes breathless more quickly than a non-smoker. Carbon monoxide combines with haemoglobin, so the blood carries less oxygen. In emphysema the alveolar walls break down, so the surface area for gas exchange is smaller. Each cause needs its effect on oxygen supply.

Resistance in three marks. Explain why doctors do not prescribe antibiotics for a sore throat caused by a virus. Antibiotics do not affect viruses, so the drug would not help. Using it would kill non-resistant bacteria in the body while any resistant bacteria survive and reproduce. Using antibiotics only when essential slows the spread of resistant strains such as MRSA.

Vaccination in four marks. Explain how a vaccine protects a child from a disease years later. The vaccine contains weakened pathogens or their antigens. These stimulate lymphocytes to produce antibodies complementary to the antigens. Memory cells are made and remain. If the real pathogen enters, memory cells respond quickly, so antibodies are made before the child becomes ill.

Must-know distinctions

Pair	Difference
Pathogen vs vector	Plasmodium causes malaria; the mosquito carries it
HIV vs AIDS	HIV is the virus; AIDS is the syndrome HIV infection may lead to
Bacteria vs viruses	Antibiotics kill bacteria; they do not affect viruses
Antigen vs antibody	Antigen is on the pathogen; antibody is the protein made against it
Phagocyte vs lymphocyte	Phagocytes engulf; lymphocytes produce antibodies
Active vs passive	Own antibodies + memory cells vs borrowed antibodies, no memory cells
Infection vs vaccination	Both give active immunity; vaccination does it without the illness

Quick self-test

1. Define a transmissible disease.
2. Give two indirect routes of transmission.
3. How does stomach acid protect the body?
4. Name the vector of malaria and the type of organism that causes it.
5. Why does spreading oil on still water reduce malaria?
6. Give three ways HIV is transmitted.
7. Which ions does the cholera toxin cause to be secreted into the small intestine?
8. Why will an antibiotic not cure influenza?
9. After bed nets were given out in a district, malaria cases fell from 3200 to 2000 a year. Calculate the percentage decrease.
10. Why does an antibody against one pathogen not protect against another?
11. Why does passive immunity last only a short time?
12. State two effects of HIV on the immune system.

Answers

1. A disease in which the pathogen can be passed from one host to another.
2. Any two: contaminated surfaces, contaminated food, animals, the air.
3. It kills pathogens in swallowed food and drink.
4. The mosquito; a parasite (a single-celled organism, Plasmodium).
5. Larvae and pupae cannot reach air at the surface and die, so fewer adult mosquitoes.
6. Any three: unprotected sex, shared needles, infected blood transfusion, mother to baby.
7. Chloride ions.
8. Influenza is caused by a virus; antibiotics do not affect viruses.
9. (3200 − 2000) ÷ 3200 × 100 = 37.5%.
10. Antibodies are complementary to specific antigens; the other pathogen’s antigens have a different shape.
11. The antibodies come from another individual and no memory cells are made, so protection ends when they break down.
12. Decreased lymphocyte numbers; reduced ability to produce antibodies.

Where marks are usually lost

• Calling the mosquito the pathogen instead of the vector.
• Listing body barriers outside the syllabus list when asked for the four named ones.
• Leaving out osmosis, or the chloride ions, from the cholera explanation.
• Writing “kills germs” for antibiotics without saying bacteria, not viruses.
• Saying bacteria “become immune” to antibiotics; immunity is a property of the host, resistance of the bacteria.
• Describing antibodies as “the same shape” as antigens instead of complementary.
• Forgetting memory cells when explaining long-term protection from a vaccine.
• Saying passive immunity involves memory cells.
• Stating that carbon monoxide damages the alveoli; it combines with haemoglobin.
• Treating a correlation in disease data as proof of cause without comment.

Official syllabus

Cambridge O Level Biology 5090 syllabus for examination in 2026, 2027 and 2028 (version 4), Topic 12: Disease and immunity, published by Cambridge Assessment International Education.