(a) the different types of pathogen that can cause communicable diseases…

The idea

A pathogen is an organism or agent that causes disease and can be passed between hosts. Learners must distinguish the main types affecting animals and plants. In animals the key groups are bacteria (for example the bacterium causing tuberculosis), viruses (for example HIV), fungi (for example ringworm) and protoctists (for example the protoctist causing malaria). In plants the key groups are fungi (for example the fungus causing black sigatoka in bananas), bacteria (for example the bacterium causing ring rot in potatoes), viruses (for example tobacco mosaic virus) and protoctists (for example the protoctist causing potato blight). Recognise that each type differs in structure and replication, so treatment differs: antibiotics act on bacteria but not on viruses. A multiple-choice question may ask which group a named example belongs to, or which type is not a pathogen of plants.

What earns the marks

• Identifies bacteria, viruses, fungi and protoctists as the main pathogen types affecting animals and plants.
• Correctly assigns a named animal disease to its pathogen group, for example tuberculosis to bacteria and HIV to viruses.
• Correctly assigns a named plant disease to its pathogen group, for example black sigatoka to fungi and tobacco mosaic virus to viruses.
• Recognises that protoctists cause disease in both animals (malaria) and plants (potato blight).
• Distinguishes pathogen groups by structure and replication, explaining why antibiotics target bacteria but not viruses.

Common mistakes and how to put them right

• Assuming all pathogens are bacteria.

  Put it right: Is that viruses, fungi and protoctists also cause communicable disease in both plants and animals.

• Treating potato blight as a fungus.

  Put it right: Is that the pathogen responsible is a protoctist.

• Believing viruses can be treated with antibiotics.

  Put it right: Is that antibiotics act on bacterial structures that viruses lack, so viruses require different control measures.

Exam tips

• Learn one named example for each pathogen group in animals and one in plants, so you can match examples to groups quickly.
• Read the question stem carefully to check whether it asks about animals, plants or both before choosing.
• Eliminate options that name a non-pathogen or a group that does not infect the stated host.