Pathogens and infections — Edexcel GCSE Biology
Types of pathogen, six named infections, how they spread and are controlled, the virus lifecycle and sexually transmitted infections.
Revision notes
5 short notes, in the order of the specification. Each one in short:
A pathogen is a disease-causing organism. The four types are viruses, bacteria, fungi and protists. Viruses are not cells and can only reproduce inside a host cell, for example HIV. Bacteria are single-celled with no nucleus, such as the cholera bacterium. A fungus causes Chalara ash dieback, and protists are single-celled with a nucleus, such as the malaria protist.
Common infections include cholera, tuberculosis, Chalara ash dieback, malaria and HIV. Cholera is caused by a bacterium and causes diarrhoea, and tuberculosis is caused by a bacterium and causes lung damage. Chalara ash dieback is caused by a fungus and damages ash trees. Malaria is caused by a protist, and HIV is a virus that destroys white blood cells.
Pathogens spread in different ways, and spread is reduced by blocking each route. Cholera spreads in contaminated water, so clean water is provided. Tuberculosis is airborne, so infected people are isolated. Chalara ash dieback spreads by airborne spores, so infected trees are destroyed. Malaria is carried by a mosquito vector, so mosquito nets are used.
The lifecycle of a virus
Triple onlyA virus can only reproduce inside a host cell, by the lytic or the lysogenic pathway. In the lytic pathway, the viral genetic material takes over the cell, new viruses are assembled and the cell bursts. In the lysogenic pathway, the viral genetic material joins the host DNA and stays dormant until it is triggered to become active.
Sexually transmitted infections (STIs) are spread by sexual contact. Chlamydia is caused by a bacterium and can cause infertility, while HIV is a virus that destroys white blood cells and can lead to AIDS. Using condoms, which act as a barrier to body fluids, screening and testing, and not sharing needles all reduce the spread.
Exam questions
6 exam-style questions (15 marks), each with its mark scheme.
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Quick check questions
Name the four types of pathogen.
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Viruses, bacteria, fungi and protists.
How is cholera spread, and how can its spread be reduced?
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Through contaminated water; provide clean water and treat sewage.
What is the vector for malaria?
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The mosquito.
Triple only: what happens to the host cell in the lytic pathway?
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It bursts open (lyses), releasing new viruses.
Name a bacterial STI and a viral STI.
Show answer
Chlamydia (bacterial) and HIV (viral).
Frequently asked questions
How do pathogens spread?
Pathogens spread in several ways: in contaminated water, like cholera; through the air in droplets or spores, like tuberculosis and Chalara ash dieback; by an animal vector, like the mosquito carrying malaria; and by sexual contact, like chlamydia and HIV. To reduce the spread, identify the route and then block it.
What is the difference between bacteria and viruses?
Bacteria are single-celled organisms with no nucleus, while viruses are not cells at all. Viruses are much smaller than bacteria and can only reproduce inside a host cell. Examples are the cholera bacterium and HIV. This is why antibiotics, which inhibit cell processes in bacteria, do not work on viruses.
How can the spread of malaria be reduced?
The spread of malaria can be reduced by controlling its vector, the mosquito, which carries the malaria protist from person to person. Draining standing water and using insecticides kill mosquitoes, while mosquito nets and insect repellent stop people being bitten. Mosquitoes do not cause malaria; the protist does.
How does HIV affect the body?
HIV is a virus that destroys white blood cells, which leads to the onset of AIDS. With fewer white blood cells, the body cannot fight off other pathogens, so people with HIV are much more likely to develop infections such as tuberculosis. HIV spreads by unprotected sex, sharing needles and contact with infected blood.
What is the difference between the lytic and lysogenic pathways?
Triple only In the lytic pathway, the virus's genetic material takes over the host cell straight away, new viruses are made and the cell bursts, releasing them. In the lysogenic pathway, the viral genetic material joins the host DNA and stays dormant, being copied each time the cell divides, until something triggers it to become active.
Written and checked against the Edexcel GCSE Biology (1BI0) specification · Updated October 2026