IGCSE Biology Diseases and immunity notes

A pathogen is a disease-causing organism — bacteria, viruses, fungi and protoctists all include pathogenic species. A transmissible disease is a disease in which the pathogen can be passed from one host to another — the key word: if it cannot transfer between hosts, it is not transmissible.

Transmission takes two broad routes. Direct contact includes transfer through blood and other body fluids — touching an infected person, sexual contact, shared needles, a blood transfusion with infected blood. Indirect transmission happens through contaminated surfaces (door handles, cutlery), contaminated food or water, animals (a mosquito or dog as carrier) and the air (droplets released in coughs and sneezes, then inhaled).

A labelled diagram to picture: the body as a castle. The skin is the wall; nose hairs are the gate filter; mucus is the sticky moat lining the airways; stomach acid is the boiling oil for anything swallowed; white blood cells are the soldiers patrolling inside. Every exam question on this list wants the defence named and its action matched to a route of entry — skin blocks entry through the surface, mucus and nose hairs block inhalation, stomach acid blocks ingestion.

The syllabus names five measures that control the spread of disease: a clean water supply, hygienic food preparation, good personal hygiene, waste disposal and sewage treatment (details of the treatment stages are not required). Each one works by cutting a transmission route from the last section — clean water and sewage treatment break the water route; hygienic food preparation breaks the contaminated-food route; personal hygiene (hand-washing) breaks the contaminated-surface and direct-contact routes; waste disposal removes the material in which pathogens breed and spread.

The importance is easiest to see in reverse: where sewage mixes with drinking water, waterborne pathogens such as the cholera bacterium spread through a whole population in days. A clean water supply plus sewage treatment is why such outbreaks are rare in countries with modern sanitation — and why they still follow natural disasters that damage those systems.

Supplement. Active immunity is defence against a pathogen by antibody production in the body. Every pathogen carries antigens — molecules on its surface, each pathogen’s antigens having specific shapes. Antibodies are proteins made by lymphocytes (a type of white blood cell) that bind to antigens; binding leads either to direct destruction of the pathogens or to marking them for destruction by phagocytes, which then engulf them.

The binding is specific: specific antibodies have complementary shapes that fit specific antigens — like a lock and key. An antibody made against measles antigen fits nothing else. This is why one infection usually protects you against that pathogen for life but not against a different one, and why the immune system must make a new antibody for every new pathogen shape.

Active immunity is gained after an infection by a pathogen (the body fights it and remembers it) or by vaccination — the next section. “Active” because the body does the work itself: its own lymphocytes make the antibodies, and memory cells keep the recipe.

Supplement. Vaccination trains the immune system without causing the disease. The process, in order: weakened pathogens or their antigens are put into the body; the antigens stimulate an immune response — lymphocytes produce antibodies specific to those antigens; and memory cells are produced that give long-term immunity. On a later encounter with the real pathogen, the memory cells recognise its antigens and the body produces antibodies fast enough to destroy the pathogen before disease develops.

This is still active immunity — the body makes its own antibodies — which is why vaccination counts as gaining active immunity. At population level, vaccination controls the spread of disease: if most people are immune, the pathogen cannot pass easily from host to host, transmission chains break, and even unvaccinated individuals are far less likely to meet the pathogen. This is how vaccination programmes have pushed some diseases close to elimination.

Supplement. Passive immunity is a short-term defence against a pathogen by antibodies acquired from another individual — the body receives ready-made antibodies instead of making its own. The two syllabus examples are maternal: antibodies cross the placenta before birth, and breast milk delivers more during feeding.

This is why breast-feeding matters for infant immunity: a newborn’s own immune system is immature, and maternal antibodies protect it against pathogens while its own active immunity develops. The protection is temporary because the borrowed antibodies are used up and broken down — and, crucially, memory cells are not produced in passive immunity, so no long-term memory of the pathogen is gained. When the antibodies are gone, so is the protection.

Supplement. Cholera is a disease caused by a bacterium transmitted in contaminated water — which is why it flares up where sewage enters drinking water, and why the clean-water and sewage-treatment measures in the Core section are its true control. The disease’s signature symptom is severe watery diarrhoea, and the mechanism is a chain the syllabus expects you to reproduce in order.

The chain: cholera bacteria in the small intestine produce a toxin; the toxin causes the intestinal lining to secrete chloride ions into the small intestine; the ion concentration in the gut rises, so water moves into the gut by osmosis (from the blood and tissues, down the water-potential gradient); the result is diarrhoea, and with the water go dehydration and loss of ions from the blood. Every arrow is a mark: toxin → chloride secretion → osmotic water movement → diarrhoea → dehydration and ion loss.