Communicable diseases — bacteria, viruses, fungi and protists
A pathogen is a microorganism that causes infectious disease. Four groups appear in GCSE biology, each with examples and methods of spread you must know.
Bacteria
Bacteria are prokaryotes that reproduce rapidly inside the body. Many do no harm, but some release toxins that damage tissues and cause symptoms.
Examples to learn:
- Salmonella — food poisoning. Spread: in food prepared in unhygienic conditions or from poultry that have not been vaccinated. Toxins cause fever, vomiting, abdominal cramps, diarrhoea.
- Gonorrhoea (caused by the bacterium Neisseria gonorrhoeae) — STI. Symptoms: yellow/green discharge, pain on urination. Treated with antibiotics, but resistant strains are now common, so prevention by barrier contraception is critical.
Viruses
Viruses are not strictly alive — they need a host cell to replicate. Inside, they take over the cell's machinery, then burst out, lysing the host cell, and spread.
Examples:
- Measles (RNA virus) — spread by inhaling droplets from sneezes/coughs. Symptoms: fever, red rash. Most children are vaccinated; serious complications include pneumonia and encephalitis.
- HIV — spread through sexual contact or sharing needles. After flu-like symptoms, the virus attacks the immune system; if untreated, the person develops AIDS. Antiretroviral drugs slow progression.
- Tobacco mosaic virus (TMV) — affects tobacco plants and many others. Causes mosaic-pattern discolouration on leaves, reducing photosynthesis and so growth.
Fungi
A fungal example:
- Rose black spot — a fungal disease. Appears as purple/black spots on rose leaves; leaves turn yellow and drop early, reducing photosynthesis. Spread by water and wind. Treatments: chemical fungicides; remove and destroy infected leaves.
Protists
A protist example:
- Malaria — caused by a Plasmodium protist. Spread: female Anopheles mosquito bite (vector). Symptoms: recurring high fever; can be fatal. Prevention: mosquito nets, insecticides, antimalarial drugs.
Reducing the spread
General principles:
- Hygiene — handwashing, food preparation hygiene
- Destroying vectors — mosquito nets, draining standing water
- Isolation of infected individuals
- Vaccination programmes (see B3.2)
⚠Common mistakes— Common mistakes / exam traps
- Treating viruses with antibiotics — they don't work; antibiotics kill bacteria.
- "Mosquitos cause malaria" — the mosquito is the vector; Plasmodium protists are the actual pathogens.
- Confusing pathogen and disease — pathogen = the microbe; disease = the resulting illness.
- HIV is a "type of AIDS" — wrong; HIV is the virus, AIDS is the late-stage syndrome.
Links
Sets up B3.2 (defence systems) and B3.3 (drug treatment, antibiotic resistance).
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