Communicable (infectious) diseases: Students should be able to explain how diseases caused by viruses, bacteria, protists and fungi are spread in animals and plants.

The idea

For animals, examples include influenza virus spread by airborne droplets, Salmonella bacteria spread by contaminated food, the protist Plasmodium spread by mosquito vectors, and the fungus Trichophyton spread by direct contact. For plants, examples include tobacco mosaic virus spread by direct contact and contaminated tools, crown gall bacteria spread through wounds, potato blight protist spread by spores in wind and water, and rose black spot fungus spread by spores in wind and water. Explaining spread means naming the pathogen group, the route and how the route lets the pathogen reach a new host.

What earns the marks

• Identify viruses, bacteria, protists and fungi as the four pathogen groups named in the statement.
• Explain that viruses spread in animals through airborne droplets, direct contact or contaminated body fluids, for example influenza.
• Explain that bacteria spread in animals through contaminated food or water, direct contact or droplets, for example Salmonella.
• Explain that protists spread in animals through vectors such as mosquitoes, for example Plasmodium causing malaria.
• Explain that fungi spread in animals through direct contact or spores, for example athlete's foot.
• Explain that plant viruses spread through direct contact or contaminated tools, for example tobacco mosaic virus.
• Explain that plant bacteria spread through wounds, water or vectors, for example crown gall disease.
• Explain that plant protists and fungi spread through spores carried by wind or water, for example potato blight and rose black spot.

Common mistakes and how to put them right

• Treating all pathogens as spreading in the same way.

  Put it right: Each pathogen group has characteristic routes, such as vectors for Plasmodium and spores for fungi.

• Confusing a vector with the pathogen itself.

  Put it right: A vector is an organism that carries and transmits the pathogen, such as a mosquito carrying Plasmodium.

• Assuming tobacco mosaic virus is spread by vectors like aphids.

  Put it right: TMV is spread by direct contact between plants and via contaminated tools.

Exam tips

• Link each named disease to its pathogen group and its transmission route in one clear sentence.
• Use comparative language when asked about more than one pathogen group, for example 'whereas fungi spread by spores, protists may be spread by vectors'.
• Include a plant example as well as an animal example when the question asks about both, so both parts of the statement are covered.