Bacterial, Fungal and Protist Diseases
In this lesson you will learn the specific diseases caused by bacteria, fungi and protists that you need to know for Edexcel GCSE Biology. For each disease, make sure you know the pathogen, how it spreads, the symptoms, and the treatment or prevention.
Bacterial Diseases
1. Salmonella (Food Poisoning)
Pathogen: Salmonella bacteria (various species, e.g. Salmonella enterica)
How it spreads:
- Eating contaminated food, especially undercooked poultry, eggs, and meat
- Bacteria are ingested and infect the gut
Symptoms:
- Fever
- Abdominal cramps
- Vomiting
- Diarrhoea
- Symptoms usually appear 12–72 hours after infection and last 4–7 days
How Salmonella causes disease:
- The bacteria produce toxins in the gut that cause inflammation, leading to the symptoms above
Prevention and control:
- Proper food hygiene: thorough cooking of poultry and eggs, handwashing, avoiding cross-contamination
- Vaccination of poultry — in the UK, chickens are vaccinated against Salmonella to reduce the presence of the bacteria in eggs and meat
- Food storage: keeping food at correct temperatures (below 5°C for refrigerated foods)
Exam tip: Remember that it is the toxins produced by Salmonella bacteria (not the bacteria themselves) that cause the symptoms of food poisoning. This is a key detail about how bacteria cause disease.
2. Gonorrhoea
Pathogen: The bacterium Neisseria gonorrhoeae
How it spreads:
- Sexual contact (unprotected vaginal, oral, or anal sex)
- It is a sexually transmitted infection (STI)
Symptoms:
- Thick yellow or green discharge from the vagina or penis
- Pain when urinating (dysuria)
- Some people (especially women) may show no symptoms, meaning they can unknowingly transmit the infection
Treatment:
- Treated with antibiotics
- However, many strains of Neisseria gonorrhoeae have developed antibiotic resistance, making treatment increasingly difficult
- Previously treated with penicillin, now often requires stronger antibiotics (e.g. ceftriaxone combined with azithromycin)
Prevention:
- Barrier contraception (condoms) — the most effective way to prevent transmission
- Regular STI screening for sexually active individuals
- Avoiding unprotected sexual contact
Exam tip: Gonorrhoea is a very common exam topic because it illustrates two important concepts: (1) it is an STI prevented by barrier contraception, and (2) it demonstrates antibiotic resistance — strains of the bacterium are becoming resistant to the antibiotics used to treat it.
3. Tuberculosis (TB)
Pathogen: The bacterium Mycobacterium tuberculosis
How it spreads:
- Airborne droplet infection — spread when an infected person coughs, sneezes, or talks
- TB is most common in overcrowded and poorly ventilated conditions
Symptoms:
- Persistent cough (often lasting more than 3 weeks)
- Coughing up blood in severe cases
- Weight loss and loss of appetite
- Fever and night sweats
- Fatigue
- TB primarily affects the lungs but can spread to other organs
Treatment:
- Treated with a long course of antibiotics (typically 6–9 months)
- It is critical that patients complete the entire course — stopping early allows the most resistant bacteria to survive and multiply, increasing the risk of drug-resistant TB
Prevention:
- The BCG vaccine (Bacillus Calmette-Guérin) provides some protection against TB
- In the UK, the BCG vaccine is offered to high-risk groups
- Improved living conditions — reducing overcrowding and improving ventilation
- Contact tracing — identifying and testing people who have been in close contact with a TB patient
Exam tip: TB is spread by airborne droplets, not by food or water. Treatment requires a long course of antibiotics — this is important because incomplete courses contribute to antibiotic resistance.
Fungal Disease
Rose Black Spot
Pathogen: The fungus Diplocarpon rosae
What it affects:
- Rose plants — this is a plant disease, not a human disease
Symptoms:
- Purple or black spots develop on the leaves
- The leaves often turn yellow around the spots
- Affected leaves drop early from the plant
- The plant has fewer leaves → less photosynthesis → reduced growth
How it spreads:
- Wind — fungal spores are carried in the air
- Water — spores spread in water splashes (rain or watering)
Treatment and prevention:
- Fungicides — chemical sprays that kill the fungus
- Removing and destroying infected leaves — by burning or putting in general waste (NOT composting, as spores can survive)
- Planting resistant varieties of roses
How the fungus causes disease:
- Fungal spores land on the leaf surface and germinate
- Hyphae (thread-like structures) penetrate the leaf cells
- The fungus feeds on the leaf tissue, destroying cells and producing the characteristic black spots
Exam tip: Rose black spot reduces photosynthesis because the plant has fewer leaves (they drop off) and the remaining leaves have less chlorophyll in the damaged areas. This links to the importance of leaves for photosynthesis.
Protist Disease
Malaria
Pathogen: Plasmodium (a protist parasite — several species, with Plasmodium falciparum being the most dangerous)
Vector: The female Anopheles mosquito
How it spreads:
- A female Anopheles mosquito bites an infected person and takes up Plasmodium with the blood meal
- The Plasmodium develops inside the mosquito
- The mosquito bites another person, injecting Plasmodium into their blood via its saliva
- The Plasmodium enters the person's liver and then red blood cells, where it reproduces
The mosquito is the vector — it carries the pathogen between hosts but is not harmed by it.
Symptoms:
- Recurring episodes (cycles) of fever — typically every 48–72 hours, corresponding to the cycle of Plasmodium bursting out of red blood cells
- Sweating and chills
- Headache and muscle pain
- Anaemia — because red blood cells are destroyed
- Severe cases can lead to organ failure and death, especially with P. falciparum
Malaria is a global killer:
- Over 200 million cases per year worldwide
- Over 600,000 deaths per year, mostly children under 5 in sub-Saharan Africa