Disease

Biology · Disease and immunity. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

Test this topic free All Biology topics

The idea

A pathogen is a disease-causing organism. A transmissible disease is one in which the pathogen can pass from one host to another. It can pass directly, through blood or other body fluids, or indirectly, through contaminated surfaces, food or water, animals or the air. The body's barriers are the skin, hairs in the nose, mucus and stomach acid.

Learn three diseases. Malaria is caused by a parasite carried by a mosquito, which is the vector. HIV is a virus that can lead to AIDS. Cholera is caused by a bacterium in contaminated water.

To control a disease, break the route of transmission. Alcohol and tobacco smoke also harm health, but they are not pathogens.

Rules to remember

Malaria: the parasite Plasmodium is passed on when a female Anopheles mosquito bites an infected person and then another person
Mosquito eggs, larvae and pupae live in still water: drain it, spray oil on it, or add fish that eat larvae; use insecticide and bed nets against adults
HIV: spread by unprotected sex, shared needles, infected blood, and from mother to baby; it may lead to AIDS
HIV control: condoms, sterile needles, screening donated blood, drugs for infected mothers, education
Cholera: bacterium in water contaminated with faeces. Its toxin makes chloride ions pass into the small intestine, water follows by osmosis → diarrhoea, dehydration, loss of ions
Cholera control: clean water supply, sewage treatment, safe waste disposal, hand washing, hygienic food preparation
Alcohol: depressant, slower reactions, less self-control, liver damage. Tobacco: nicotine is addictive; tar causes lung cancer and bronchitis; carbon monoxide lowers oxygen carried by haemoglobin; smoking in pregnancy lowers birth weight

Common mistake

The mosquito does not cause malaria. It is the vector that carries the pathogen; the pathogen is the parasite Plasmodium. In the same way, HIV is the virus and AIDS is the condition it may lead to.

Worked example

Small fish that feed on insect larvae are put into the ponds near a town. A year later there are far fewer cases of malaria. Explain why.

Mosquitoes lay their eggs in still water, and the larvae live there.
The fish eat the larvae, so fewer larvae grow into adult mosquitoes.
With fewer adult mosquitoes, fewer bite an infected person and then carry the parasite to a healthy person.

Answer: The fish break the life cycle of the vector at the larva stage, so the parasite is transmitted less often.

Ask about this

Type what you are stuck on. The helper finds the explanation, worked examples and the notes to read, free and instant.

Read the full notes

Keep going

← Blood Antibiotics →Take a Biology test