Edexcel IGCSE Biology · Spec 1.4
Pathogens and Viruses
What a pathogen is, the four types (bacteria, fungi, protoctists and viruses), the structure of a virus, and why viruses are not truly living.
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Pathogens and Viruses
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Explained
What makes something a pathogen, and why viruses are odd
A pathogen is a microorganism or infectious agent that causes disease. The word describes an effect, not a group, which is the first thing to be clear about. Fungi, bacteria, protoctists and viruses can all contain pathogens, and most members of those groups are harmless.
So being a bacterium does not make something a pathogen, and the four groups named in the specification are worth learning because a question can ask which groups may include them.
Viruses are not cells
A virus is far smaller than a bacterium and has no cellular structure at all. There is no nucleus, no cytoplasm, no cell membrane and no organelles.
It consists of a protein coat around a single type of nucleic acid, either DNA or RNA, and never both.
This is why viruses are not classed as living organisms. They do not respire, grow, excrete or feed. The only characteristic of living things they show is reproduction, and even that they cannot do alone.
Why they need a host
A virus has none of the machinery for making proteins or copying nucleic acid. It has to take over a living cell and use that cell's machinery instead.
The virus enters a host cell, its nucleic acid directs the cell to make new virus particles, and the cell eventually bursts and releases them to infect others. That bursting is what damages the tissue and causes the symptoms.
Viruses can infect every type of living organism, including bacteria, and each virus can usually only infect particular cells of particular species.
The named examples
Influenza virus causes flu, infecting the cells lining the airways.
HIV infects lymphocytes, the white blood cells that make antibodies. Destroying them gradually weakens the immune system, which can lead to AIDS, at which point ordinary infections become dangerous. Note the distinction: HIV is the virus and AIDS is the condition that can follow.
Tobacco mosaic virus infects plants and prevents chloroplasts from forming in the leaf cells, giving a patchy discoloured appearance. With fewer chloroplasts the plant photosynthesises less, so it grows poorly.
What the mark scheme accepts and rejects
An Edexcel International GCSE Biology mark scheme for a question on how white blood cells fight pathogens lists six credited points, and then adds a single blunt note: no credit for phagocytes release antibodies, or lymphocytes engulf.
Swapping the two cells over scores nothing, even though every individual word in the sentence is a correct biology term. It is worth fixing the pairing now: phagocytes engulf and digest, lymphocytes produce antibodies.
The multiple choice questions in these papers make the same point about classification. One asks which group Euglena belongs to, and the mark scheme explains each wrong option by saying simply that Euglena is not a bacterium, not a fungus and not a virus. It is a protoctist.
Knowing which group an organism belongs to is treated as recall you either have or do not, so the named examples in the specification are worth learning as a list with their groups attached.
Why antibiotics do not work on viruses
Antibiotics work by damaging structures that bacteria have and human cells do not, such as the bacterial cell wall.
A virus has no such structures, and while it is reproducing it is inside one of your own cells. There is nothing for the antibiotic to attack that is not also part of you. This is why flu cannot be treated with antibiotics, and why prescribing them for viral infections does nothing except encourage resistance in the bacteria that happen to be present.
Spec 1.4
What you need to know
- Define a pathogen and list the four types
- Describe the range and structure of viruses
- Explain how viruses reproduce inside living host cells
Active recall
Quick check
Answer each question before opening the answer.
What are the four types of pathogen?
Bacteria, viruses, fungi and protoctists.
Why are viruses not considered living organisms?
They have no cellular structure and cannot carry out life processes or reproduce on their own — they can only replicate inside a host cell.
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