Viruses and pathogens

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In short

A pathogen is an organism or particle that causes disease, such as a fungus, bacterium, protoctist or virus. Viruses are not living organisms: they are particles smaller than bacteria, with no cellular structure, a protein coat and one type of nucleic acid, DNA or RNA. They are parasitic and can reproduce only inside living cells.

A pathogen is an organism or particle that causes disease. Pathogens may include fungi, bacteria, protoctists or viruses.

Viruses

Viruses are not living organisms. They are small particles, smaller than bacteria, and they have a wide variety of shapes and sizes.

They have no cellular structure.
They have a protein coat and contain one type of nucleic acid, either DNA or RNA.
They are parasitic: they can reproduce only inside living cells.
They infect every type of living organism.

A bacterium with a tiny virus beside it at the same scale, and a magnified virus particle labelled protein coat and nucleic acid (DNA or RNA). — A virus is much smaller than a bacterium. It has a protein coat around one type of nucleic acid (DNA or RNA) and no cellular structure.

Examples of viruses
Virus	Disease or effect
Tobacco mosaic virus	Discolours the leaves of tobacco plants by preventing the formation of chloroplasts
Influenza virus	Causes flu
HIV	Causes AIDS

Examples of pathogens from this topic
Type of pathogen	Example	Disease
Virus	HIV	AIDS
Bacterium	Pneumococcus	Pneumonia
Protoctist	Plasmodium	Malaria

Exam tip:

Say that viruses 'reproduce only inside living cells' and that they have 'no cellular structure'. These are the usual mark-scheme points for why a virus is not a living organism.

Quick check

Give any four characteristics shared by all living organisms.
Show answer
Any four from: nutrition, respiration, excretion, response to surroundings, movement, control of internal conditions, reproduction, growth and development.
What are fungal cell walls made of?
Show answer
Chitin.
What is the name for the thread-like structures that make up a fungal mycelium?
Show answer
Hyphae.
Give two features of a bacterial cell.
Show answer
Any two from: cell wall, cell membrane, cytoplasm, plasmids, circular chromosome of DNA, no nucleus.
Why are viruses not classed as living organisms?
Show answer
They have no cellular structure and can reproduce only inside living cells.

Written and checked against the Edexcel IGCSE Science Double Award (4SD0) specification · Updated October 2026

Frequently asked questions

What are the eight characteristics of living organisms?

The eight characteristics are nutrition, respiration, excretion, response to surroundings, movement, control of internal conditions, reproduction, and growth and development. All living organisms show every one of them, although in different ways. A plant, for example, moves by growing towards light, while an animal can move from place to place.

What is the difference between excretion and egestion?

Excretion is removing the waste products made by chemical reactions in cells, such as carbon dioxide. Egestion is removing undigested food from the gut. Only excretion is one of the characteristics of living organisms, so in an exam do not use the two words as if they mean the same thing.

What is the difference between prokaryotic and eukaryotic cells?

Prokaryotic cells, such as bacteria, have no nucleus: their DNA is a circular chromosome free in the cytoplasm, often with extra rings called plasmids. Eukaryotic cells, found in plants, animals, fungi and protoctists, have their DNA enclosed in a nucleus. Bacteria are also microscopic and single-celled, while many eukaryotes are multicellular.

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