Pathogens and communicable disease

Pathogens are microorganisms that cause infectious (communicable) diseases in animals and plants: viruses, bacteria, fungi and protists. They spread by direct contact, by water, through the air and by vectors. Knowing how a disease spreads tells us how to stop it.

Key facts

  • Viruses live and reproduce inside cells, which damages them. Bacteria reproduce rapidly and may release toxins that damage tissues and make us feel ill.
  • Viral diseases: measles (fever and a red skin rash; spread in droplets; prevented by vaccination), HIV (a flu-like illness, then damage to the immune system; spread by sexual contact and body fluids; controlled by antiretroviral drugs) and tobacco mosaic virus (discoloured leaves, less photosynthesis).
  • Bacterial diseases: salmonella food poisoning (fever, cramps, vomiting and diarrhoea, from undercooked food; poultry are vaccinated) and gonorrhoea (a sexually transmitted disease, once treated with penicillin, but many strains are now resistant).
  • Fungal and protist diseases: rose black spot (black spots on leaves; fungicides and removing leaves) and malaria (a protist carried by mosquitoes, which are vectors; repeated fevers).
  • The spread of disease can be reduced by hygiene, isolating infected people, destroying vectors and vaccination.
  • Bacteria divide by binary fission, as often as every 20 minutes: number = starting number × 2^(number of divisions).
  • Antibiotics and antiseptics can be compared by the clear zone around a disc on a plate of bacteria: area = πr². Plates are kept at 25 °C in schools so harmful pathogens are less likely to grow.

Worked examples

Example 1

A bacterium divides every 30 minutes. Starting from one bacterium, how many are there after 3 hours?

  1. 3 hours = 180 minutes. 180 ÷ 30 = 6 divisions
  2. The number doubles each time. 2^6 = 64

Answer: 64

Example 2

An antibiotic disc produced a clear zone 18 mm across. Calculate the area of the clear zone.

  1. The radius is half the diameter. 18 ÷ 2 = 9 mm
  2. Area = πr². π × 9² = 254.5 mm²
  3. A bigger clear zone means the antibiotic stopped more bacteria growing.

Answer: 254.5 mm²

Example 3

Explain how malaria is spread and give two ways to stop it spreading.

  1. Malaria is caused by a protist.
  2. Mosquitoes carry it from person to person when they feed on blood: they are vectors.
  3. Draining standing water stops mosquitoes breeding.
  4. Mosquito nets and insect repellent stop people being bitten.

Answer: a protist is carried by mosquitoes (vectors); stop mosquitoes breeding and use mosquito nets

Example 4

Why can measles not be treated with antibiotics, and how is it prevented?

  1. Measles is caused by a virus, which lives inside the body's cells.
  2. Antibiotics kill bacteria, not viruses.
  3. Vaccinating young children prevents measles.
  4. Infected people are isolated, because it spreads in droplets from coughs and sneezes.

Answer: measles is a virus and antibiotics only kill bacteria; children are vaccinated