Cambridge O Level Biology 5090: Disease and immunity (5090)

This study guide teaches Topic 12, Disease and immunity, for Cambridge O Level Biology 5090. It is aligned to the Cambridge O Level Biology 5090 syllabus for examination in 2026, 2027 and 2028, and covers every learning outcome in sections 12.1 Disease, 12.2 Antibiotics and 12.3 Immunity. The 5090 syllabus is not tiered, so every candidate studies all of it.

Use it with the revision notes for this topic and the practice questions. The course hub lists every topic, and the printable checklist lets you tick off each outcome. To find your weak spots quickly, try one of the free 10-minute diagnostics.

What this topic covers

Syllabus ref	What you must be able to do
12.1.1–12.1.3	Define pathogen and transmissible disease; direct and indirect transmission
12.1.4	Body barriers: skin, nose hairs, mucus, stomach acid
12.1.5–12.1.7	Mosquito as a vector; malaria; controlling the mosquito through its life cycle
12.1.8–12.1.11	HIV: a viral pathogen, transmission, link to AIDS, control
12.1.12–12.1.14	Cholera: spread, control, how the toxin causes diarrhoea
12.1.15–12.1.16	Effects of excess alcohol and of tobacco smoke
12.2.1–12.2.4	Drugs, antibiotics, viruses, limiting antibiotic resistance (MRSA)
12.3.1–12.3.11	Antigens, antibodies, active and passive immunity, vaccination, HIV and the immune system

12.1 Disease

Pathogens and transmission (12.1.1–12.1.3)

• A pathogen is a disease-causing organism.
• A transmissible disease is a disease in which the pathogen can be passed from one host to another.

Pathogens are transmitted in two ways:

Route	Meaning	Examples
Direct contact	Pathogen passes straight from one person to another	Blood or other body fluids, e.g. HIV in blood
Indirect	Pathogen passes via something in between	Contaminated surfaces or food, animals (e.g. mosquitoes), the air (droplets from coughs and sneezes), contaminated water

Body barriers (12.1.4)

The syllabus limits this to four barriers:

Barrier	How it stops pathogens
Skin	A tough outer layer that pathogens cannot easily cross unless it is cut
Hairs in the nose	Trap dust and larger particles carrying pathogens
Mucus	Sticky; traps pathogens in the airways (it is then moved away by cilia)
Stomach acid	Hydrochloric acid kills most pathogens swallowed in food and drink

Mucus and cilia are covered in detail in Human gas exchange.

Malaria and the mosquito (12.1.5–12.1.7)

A vector is an organism that carries a pathogen from one host to another without itself suffering the disease. The mosquito is the vector of malaria.

The malarial pathogen, Plasmodium, is a single-celled parasite: it lives in or on another organism (the host) and harms it. Transmission works like this:

1. A female mosquito bites an infected person and takes in blood containing the parasite.
2. The parasite develops inside the mosquito.
3. When the mosquito bites a second person, the parasite passes from its saliva into that person’s blood.
4. The parasite travels to the liver, multiplies, then infects and destroys red blood cells.

Only female mosquitoes feed on blood, so only females transmit malaria.

Control through the life cycle. The mosquito lays its eggs in still water. The eggs hatch into larvae, which become pupae; both live in water but breathe air at the surface. The adult then emerges and flies. Each stage gives a control method:

Stage	Control method	Why it works
Eggs, larvae, pupae	Drain or cover areas of still water	Removes breeding sites
Larvae, pupae	Spread a thin layer of oil on the water	They cannot reach air at the surface, so they die
Larvae	Put fish that eat larvae into ponds; spray bacteria that kill larvae	Biological control
Adults	Spray insecticide on walls; sleep under insecticide-treated nets; use repellent	Kills adults or stops them biting

HIV and AIDS (12.1.8–12.1.11)

Human immunodeficiency virus (HIV) is a viral pathogen. It is transmitted in body fluids:

• unprotected sexual intercourse with an infected person
• sharing needles used to inject drugs
• transfusion of infected blood
• from mother to baby during pregnancy, at birth or in breast milk

HIV infection may lead to Acquired Immune Deficiency Syndrome (AIDS). HIV damages the immune system (see 12.3.11), so the person catches other infections that a healthy person would fight off.

Control methods:

• using condoms
• not sharing needles; supplying sterile needles
• screening donated blood for HIV
• testing, so infected people know their status and can be treated
• antiviral drugs, which reduce the amount of virus and the chance of passing it on, including from mother to baby
• education about how HIV spreads

There is no vaccine for HIV, so vaccination is not a control method here.

Cholera (12.1.12–12.1.14)

Cholera is a disease caused by a bacterium and transmitted in contaminated water. Faeces from an infected person enter water; someone drinks the water or eats food washed in it.

Measure	How it limits cholera
Clean water supply	Water is treated, so bacteria are killed before people drink it
Hygienic food preparation	Washing hands and using clean water stops bacteria reaching food
Good personal hygiene	Washing hands after using the toilet removes bacteria from faeces
Waste disposal	Stops waste contaminating water and attracting flies
Sewage treatment	Faeces are treated before release, so bacteria do not reach drinking water

You do not need the stages of sewage treatment.

How cholera causes diarrhoea. This is a chain; learn every link:

1. The bacterium produces a toxin in the small intestine.
2. The toxin causes chloride ions to be secreted into the small intestine.
3. This lowers the water potential of the gut contents.
4. Water moves out of the blood into the gut by osmosis.
5. Result: watery diarrhoea, dehydration, and loss of ions from the blood.

Osmosis and water potential are covered in Movement into and out of cells.

Worked example 1. Explain why cholera can kill a person within days.

The toxin causes chloride ions to be secreted into the small intestine. Water follows by osmosis into the gut, so large volumes are lost as diarrhoea. The blood loses water and ions, and the body becomes severely dehydrated.

Alcohol and tobacco (12.1.15–12.1.16)

Effects of excessive alcohol:

• Depressant: slows the nervous system.
• Reduced self-control: risky or aggressive behaviour.
• Slower reaction times: dangerous when driving or using machinery.
• Liver damage: the liver breaks down alcohol, and long-term heavy drinking damages it.
• Social implications: accidents, violence, family breakdown, lost work, cost to health services.

Tobacco smoke has three major toxic components:

Component	Effect
Nicotine	Addictive; increases heart rate and blood pressure
Tar	Contains carcinogens (cause cancer); coats the airways and stops cilia working, so mucus builds up
Carbon monoxide	Combines with haemoglobin in red blood cells, so less oxygen is carried

Smoking is strongly associated with:

• bronchitis: inflamed airways, mucus collects, persistent cough and infections
• emphysema: alveolar walls break down, so the surface area for gas exchange falls and the person is breathless
• lung cancer: carcinogens in tar cause uncontrolled cell division
• heart disease: nicotine and carbon monoxide damage the heart and blood vessels
• reduced birth weight of the baby when the mother smokes during pregnancy: carbon monoxide reduces oxygen reaching the fetus

The syllabus says “strong association”, so describe a link.

12.2 Antibiotics

• A drug is any substance taken into the body that modifies or affects chemical reactions in the body.
• Antibiotics are drugs used to treat bacterial infections. They kill bacteria but do not affect viruses, so they do not cure colds, flu or HIV.

Antibiotic resistance. In a population of bacteria, a few may be resistant to an antibiotic. When the antibiotic is used, the non-resistant bacteria die but the resistant ones survive and reproduce. With repeated use the resistant type becomes common. MRSA is a bacterium that is resistant to many commonly used antibiotics, so infections with it are hard to treat.

The more an antibiotic is used, the more chances resistant bacteria have to take over. So resistance is minimised by using antibiotics only when essential: not for viral infections, and not for minor infections the body can clear.

12.3 Immunity

Antigens and antibodies (12.3.1–12.3.4)

• Active immunity is defence against a pathogen by antibody production in the body.
• Each pathogen has its own antigens, which have specific shapes.
• Antibodies are proteins (see Biological molecules) that bind to antigens. This leads to:
  • direct destruction of the pathogen, or
  • marking the pathogen for destruction by phagocytes.
• Specific antibodies have complementary shapes which fit specific antigens. An antibody against one pathogen does not fit the antigens of a different pathogen.

Active immunity and vaccination (12.3.5–12.3.7)

Active immunity is gained after an infection by a pathogen, or by vaccination.

Vaccination:

1. Weakened pathogens or their antigens are given (often by injection).
2. The antigens stimulate an immune response by lymphocytes, which produce antibodies.
3. Memory cells are produced that give long-term immunity.

If the real pathogen enters later, memory cells respond quickly, so antibodies are made before the person becomes ill.

Vaccination and spread. A vaccinated person does not get ill and is unlikely to pass the pathogen on. When a large proportion of a population is vaccinated, the pathogen has few hosts to pass between, so it cannot spread far. This also protects people who cannot be vaccinated.

Worked example 2. In a region, vaccination coverage against a disease rose from 40% to 90% over ten years. Cases per year fell from 1250 to 75. Calculate the percentage decrease in cases and explain it.

percentage decrease = (1250 − 75) ÷ 1250 × 100
                    = 1175 ÷ 1250 × 100
                    = 94%

Explanation: more people had memory cells, so fewer became ill, and the pathogen had fewer chances to be transmitted. The data show a correlation; other factors (such as hygiene) may also have changed.

Worked example 3. After a first dose of a vaccine, antibody concentration peaked at 12 arbitrary units. After a second dose it peaked at 180 arbitrary units. How many times higher is the second peak? Explain the difference.

180 ÷ 12 = 15 times higher. The first dose produced memory cells. At the second dose, memory cells respond quickly, so antibodies are made faster and in far greater amounts.

Passive immunity (12.3.8–12.3.10)

Passive immunity is a short-term defence against a pathogen by antibodies acquired from another individual. The syllabus limits this to two routes:

• across the placenta, from mother to fetus
• in breast milk, from mother to baby

Breast-feeding matters because a newborn’s own immune system is not yet producing many antibodies. Antibodies in breast milk protect the baby from pathogens the mother has met, during the weeks when it is most vulnerable.

Memory cells are not produced in passive immunity. The baby’s own lymphocytes were not stimulated, so when the borrowed antibodies break down, the protection ends.

	Active	Passive
Antibodies made by	Your own lymphocytes	Another individual
Memory cells	Yes	No
Duration	Long-term	Short-term
Examples	Infection, vaccination	Placenta, breast milk

HIV and the immune system (12.3.11)

HIV infects and destroys lymphocytes. The outcome is limited to:

• decreased lymphocyte numbers
• reduced ability to produce antibodies

So the immune system is weakened, other pathogens are not destroyed, and the person may develop AIDS.

Common errors

• Saying antibiotics kill viruses. They do not.
• Saying the antibiotic “causes” resistance. Resistant bacteria were already present.
• Writing that antibodies “eat” pathogens. Phagocytes engulf; antibodies bind.
• Saying antibodies are the same shape as antigens. They are complementary.
• Giving memory cells to passive immunity.
• Leaving chloride ions or osmosis out of the cholera chain.
• Calling the mosquito the pathogen. It is the vector.

Next steps

Condense this with the revision notes, then test yourself with the practice questions.

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.