Describe a pathogen as a disease-causing organism, including viruses,…

The idea

A pathogen is any organism or agent that causes disease in a host. The definition turns on two ideas: it must be capable of causing disease, and it must do so in the organism it infects. The specification names four groups. Viruses are not cells: they are genetic material inside a protein coat and reproduce only inside a host cell, so they damage the cells they invade. Bacteria are single-celled prokaryotes; some release toxins or damage tissue directly. Fungi, such as the species causing Chalara ash dieback, grow hyphae into tissue and cause lesions and leaf loss. Protists are single-celled eukaryotes; the malaria parasite is one example. A multiple-choice question may ask which group a named pathogen belongs to, or which statement correctly defines a pathogen. Read each option and check both parts: does it cause disease, and is the group correct?

What earns the marks

• A pathogen is defined as an organism or agent that causes disease.
• Viruses are pathogens but are not cells; they reproduce inside host cells and damage them.
• Bacteria are single-celled prokaryotes; some cause disease by releasing toxins or damaging tissue.
• Fungi are pathogens that can grow hyphae through plant tissue, causing lesions and leaf loss.
• Protists are single-celled eukaryotes, and some, such as the malaria parasite, are pathogens.
• The four named pathogen groups in the specification are viruses, bacteria, fungi and protists.

Common mistakes and how to put them right

• Thinking all microorganisms are pathogens.

  Put it right: Most bacteria and fungi are harmless or beneficial, so the disease-causing role must be stated.

• Classifying viruses as bacteria.

  Put it right: Viruses are not cells and cannot reproduce outside a host, so they belong in their own group.

• Assuming protists are always harmless.

  Put it right: Some protists, such as the malaria parasite, are pathogens that damage blood and liver.

Exam tips

• Underline the two key words in the question, such as 'pathogen' and 'group', before choosing an option.
• Eliminate options that describe a non-infectious cause of disease, such as diet or genetics.
• Check that the named example really belongs to the group given in the option.