Infectious diseases

Biology AS · Infectious diseases. 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

An infectious disease is caused by a pathogen and is transmissible: it can pass from one host to another. For cholera, malaria, tuberculosis (TB) and HIV/AIDS you must know the name and type of the pathogen, how it is transmitted and how the disease can be prevented and controlled.

Prevention works by breaking the route of transmission. So the method depends on the route: clean water for cholera, control of mosquitoes for malaria, finding and treating cases for TB, and safer sex and safe blood for HIV.

Control is also a social and economic problem. Poverty, overcrowding, poor sanitation, lack of health workers and the cost of drugs all make these diseases harder to control.

Rules to remember

Cholera: the bacterium Vibrio cholerae; spread by water or food contaminated with faeces.
Malaria: protoctists of the genus Plasmodium (falciparum, malariae, ovale, vivax); spread by the bite of a female Anopheles mosquito, the vector.
TB: the bacteria Mycobacterium tuberculosis (airborne droplets) and Mycobacterium bovis (also from infected cattle, in unpasteurised milk or undercooked meat).
HIV/AIDS: the human immunodeficiency virus; spread in body fluids by unprotected sex, shared needles, blood transfusion, and from mother to child across the placenta, at birth or in breast milk.
Antibiotics can treat cholera and TB (bacteria) but not HIV (a virus).
Vaccines: the BCG vaccine gives some protection against TB; there is no vaccine in use for HIV; a highly effective malaria vaccine is hard to make because the parasite has many stages and antigens, and the vaccines now used give only partial protection.

Common mistake

Calling the mosquito the cause of malaria. The mosquito is the vector; the pathogen is Plasmodium, a protoctist and not a bacterium or a virus.

Worked example

Four control measures are: boiling drinking water, sleeping under insecticide-treated nets, using condoms, and pasteurising milk. Match each one to cholera, malaria, TB or HIV.

Boiling water kills bacteria carried in water: cholera.
Nets stop Anopheles mosquitoes biting at night: malaria.
Condoms stop the exchange of body fluids during sex: HIV.
Pasteurising milk kills Mycobacterium bovis from infected cattle: TB.

Answer: Boiling water: cholera. Nets: malaria. Condoms: HIV. Pasteurised milk: TB.

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