Pathogens may be viruses, bacteria, protists or fungi.

The idea

Pathogens belong to four broad groups: viruses, bacteria, protists and fungi. Each group has a different cell structure and method of causing harm, so treatment differs. Viruses are not cells; they reproduce inside host cells and destroy them. Bacteria are single-celled organisms that can divide rapidly and may release toxins. Protists are mostly single-celled eukaryotes, and some are parasites transmitted by vectors. Fungi may be single-celled or multicellular and can grow on and penetrate host tissue. Examples include influenza virus, the bacterium causing cholera, the protist causing malaria and the fungus causing rose black spot.

What earns the marks

• Lists viruses, bacteria, protists and fungi as the four pathogen groups.
• Describes viruses as non-cellular agents that reproduce inside host cells.
• Describes bacteria as single-celled organisms that divide rapidly and may release toxins.
• Describes protists as eukaryotic microorganisms, some of which are parasites spread by vectors.
• Describes fungi as organisms that can grow on and penetrate host tissue, causing damage.
• Classifies a named pathogen correctly into one of the four groups.

Common mistakes and how to put them right

• Treating viruses as cells.

  Put it right by stating they are non-cellular and reproduce only inside host cells.

• Assuming all bacteria are pathogens.

  Put it right by noting many are harmless or beneficial.

• Placing malaria in the wrong group.

  Put it right by identifying the malarial pathogen as a protist.

Exam tips

• Learn one named example for each of the four groups so classification questions are quick.
• Use structural features, such as presence of a cell or mode of reproduction, to justify the group.
• If asked to compare groups, give one similarity and one difference rather than two separate descriptions.