Communicable diseases

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

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The idea

A pathogen is a microorganism that causes disease. The four types are viruses, bacteria, protists and fungi. They infect animals and plants, and they spread by direct contact, by water or by air.

Bacteria and viruses reproduce quickly inside the body. Bacteria may produce poisons called toxins that damage tissues and make us feel ill. Viruses live and reproduce inside cells, and this damages the cells.

For each named disease, learn four things: the type of pathogen, how it spreads, the symptoms, and how it is prevented or treated. Spread is reduced by hygiene, by isolating infected people, by destroying vectors and by vaccination.

Rules to remember

Measles (virus): spread by droplets from coughs and sneezes. Fever and a red skin rash. Can be fatal, so most young children are vaccinated.
HIV (virus): spread by sexual contact or exchange of body fluids such as blood. Flu-like illness at first, then it attacks immune cells. Controlled with antiretroviral drugs; late stage is AIDS.
Tobacco mosaic virus: mosaic pattern of discoloured patches on leaves of many plants, including tomatoes. Less photosynthesis, so less growth.
Salmonella (bacteria): from food that is undercooked or prepared in unhygienic conditions. Fever, abdominal cramps, vomiting, diarrhoea. In the UK poultry are vaccinated.
Gonorrhoea (bacteria): sexually transmitted. Thick yellow or green discharge and pain when urinating. Treated with antibiotics, but many strains resist penicillin; a condom prevents spread.
Rose black spot (fungus): purple or black spots on leaves, which turn yellow and drop early. Spread by water or wind. Use fungicides and remove and destroy affected leaves.
Malaria (protist): carried by mosquitoes, which are the vectors. Repeated fevers; can be fatal. Stop mosquitoes breeding and use mosquito nets.

Common mistake

Students match the disease to the wrong type of pathogen, for example calling malaria a virus or a bacterium. Malaria is caused by a protist; the mosquito only carries it.

Worked example

A child has a fever and a red skin rash. A doctor says the child caught the disease by breathing in droplets from another child's sneeze. Name the disease and say why antibiotics will not cure it.

Fever with a red skin rash, spread by droplets in the air: this matches measles.
Measles is caused by a virus.
Antibiotics kill bacteria only. They cannot kill viruses, which live inside body cells.

Answer: Measles. It is caused by a virus, so antibiotics do not work; vaccination prevents it.

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Infection And Response — Worksheet

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