B9: Pathogens and Disease
Communicable diseases caused by pathogens — how microorganisms cause disease, how they spread, and how we can prevent their spread.
B9 Pathogens And Disease
What Are Pathogens?
Pathogens are microorganisms that cause disease. They are also called communicable (infectious) diseases because they can be passed from one person to another.
Pathogens enter the body, reproduce rapidly, and cause symptoms of disease. There are four main types of pathogen that you need to know:
- Bacteria — very small, single-celled organisms that reproduce rapidly inside the body
- Viruses — even smaller than bacteria, they live and reproduce inside living cells, damaging and destroying them
- Fungi — some are single-celled, others have a body made of thread-like structures called hyphae
- Protists — single-celled eukaryotic organisms
Not all microorganisms are pathogens. Many bacteria and fungi are harmless or even beneficial. Pathogens are the ones that cause disease.
Four Types of Pathogens
| Pathogen Type | Features | Example Disease | How It Causes Harm |
|---|---|---|---|
| Bacteria | Very small (0.2–2 µm), single-celled, reproduce rapidly by binary fission | Salmonella, Gonorrhoea | Produce toxins that damage tissues and make you feel ill |
| Viruses | Even smaller (0.02–0.3 µm), not living cells, have a protein coat and genetic material | HIV/AIDS, Measles, Tobacco mosaic virus | Take over cells, reproduce inside them, then burst out — destroying the cell |
| Fungi | Single-celled or multicellular, grow thread-like hyphae | Rose black spot | Hyphae grow into and through living tissue, digesting and absorbing nutrients |
| Protists | Single-celled eukaryotic organisms | Malaria | Live in the body and reproduce inside red blood cells, causing damage |
How Pathogens Are Spread
Pathogens can be spread in four main ways: water, air, direct contact, and vectors.
| Method of Spread | Description | Example |
|---|---|---|
| Water | Pathogens are carried in contaminated water. Drinking or bathing in it causes infection. | Cholera bacteria in contaminated water supplies |
| Air | Pathogens are carried in droplets produced when an infected person sneezes, coughs, or talks. | Measles virus in airborne droplets |
| Direct contact | Pathogens are transferred by touching an infected person, contaminated surfaces, or through bodily fluids. | Gonorrhoea through sexual contact; HIV through bodily fluids |
| Vectors | A vector is an organism that carries and transmits a pathogen to another organism without getting the disease itself. | Mosquitoes carry the malaria protist from person to person |
Preventing the Spread of Pathogens
The spread of pathogens can be reduced or prevented by: hygiene, vaccination, destroying vectors, and isolating infected individuals.
- Hygiene — washing hands thoroughly, disinfecting surfaces, using hand sanitisers, proper food preparation and storage
- Vaccination — immunising people against diseases so they cannot catch or spread them (covered in B11)
- Destroying vectors — eliminating the organisms that carry pathogens, e.g. using insecticides to kill mosquitoes, draining standing water where they breed
- Isolating infected individuals — keeping people with infectious diseases away from healthy people to prevent transmission
Specific Diseases in Detail
Salmonella (Food Poisoning)
Salmonella is a bacterial pathogen that causes food poisoning. It is spread by eating food contaminated with Salmonella bacteria.
- Pathogen type: Bacterium
- How it is spread: Eating food contaminated with Salmonella bacteria — commonly undercooked poultry, eggs, or unpasteurised milk. Also spread by not washing hands after handling raw meat.
- Symptoms: Fever, abdominal cramps, vomiting, and diarrhoea. These are caused by toxins produced by the bacteria.
- Prevention: Proper food hygiene — cooking food thoroughly (especially poultry), avoiding cross-contamination between raw and cooked foods, refrigerating food properly, washing hands after handling raw meat. In the UK, poultry are vaccinated against Salmonella to reduce the risk.
Gonorrhoea
Gonorrhoea is a sexually transmitted infection (STI) caused by bacteria. It is spread by unprotected sexual contact.
- Pathogen type: Bacterium
- How it is spread: Through unprotected sexual contact (direct contact with bodily fluids)
- Symptoms: Pain when urinating, thick yellow or green discharge from the penis or vagina. Some people have no symptoms, so they can pass it on without knowing.
- Treatment: Antibiotics (e.g. penicillin). However, some strains are now resistant to antibiotics.
- Prevention: Using barrier methods such as condoms during sexual contact. Getting tested and treated early, and informing sexual partners.
Malaria
Malaria is caused by a protist pathogen and is spread by mosquitoes acting as vectors. It is a major killer in tropical regions.
- Pathogen type: Protist
- How it is spread: By female Anopheles mosquitoes acting as vectors. The mosquito carries the protist in its mouthparts and passes it into the bloodstream when it bites a person.
- Symptoms: Recurrent episodes of fever, shaking, chills, and sweating. The protist enters red blood cells and multiplies, causing them to burst open — this leads to anaemia and can be fatal.
- Prevention: Controlling the mosquito vector — using insecticide-treated mosquito nets, using insect repellent, draining standing water where mosquitoes breed, and spraying insecticides. Taking antimalarial drugs before travelling to at-risk areas can also provide protection.
Rose Black Spot
Rose black spot is a fungal disease that affects roses. It causes black or purple spots on leaves, which turn yellow and drop off early.
- Pathogen type: Fungus
- How it is spread: Fungal spores are spread by wind and by water splashing from infected leaves onto healthy leaves
- Symptoms: Black or purple spots on leaves, leaves turning yellow and dropping off. This reduces the plant's ability to photosynthesise, weakening the plant and reducing growth.
- Treatment and prevention: Spraying with fungicides (chemicals that kill fungi). Removing and destroying infected leaves to prevent spores from spreading. Avoiding overhead watering (which creates water splashes that spread spores). Growing resistant varieties of roses.
| Disease | Pathogen Type | Method of Spread | Key Symptoms | Prevention / Treatment |
|---|---|---|---|---|
| Salmonella | Bacterium | Contaminated food | Fever, vomiting, diarrhoea, abdominal cramps | Good food hygiene, cooking thoroughly, vaccination of poultry |
| Gonorrhoea | Bacterium | Direct sexual contact | Pain urinating, discharge | Condoms (barrier method), antibiotics |
| Measles | Virus | Airborne droplets | Fever, red skin rash, complications can be serious | Vaccination (MMR) |
| HIV/AIDS | Virus | Direct contact (bodily fluids) | Flu-like illness, attacks immune system | Condoms, not sharing needles, antiretroviral drugs |
| Tobacco mosaic virus | Virus | Direct contact between plants | Mosaic pattern on leaves, stunted growth | Good hygiene, resistant varieties, destroying infected plants |
| Rose black spot | Fungus | Wind and water (spores) | Black/purple spots on leaves, yellowing, leaf drop | Fungicides, removing infected leaves |
| Malaria | Protist | Vector (mosquito) | Recurrent fever, chills, sweating | Mosquito nets, insecticides, draining standing water |
Worked Examples
Worked Example 1: Identifying the Type of Pathogen
Question: A disease is caused by an organism that reproduces inside living cells and destroys them. It has a protein coat surrounding genetic material. What type of pathogen causes this disease? Explain your answer.
Solution:
The pathogen is a virus.
- Viruses reproduce inside living cells (they are obligate intracellular parasites)
- They destroy the host cell when they burst out of it after replicating
- Viruses have a protein coat that surrounds their genetic material (DNA or RNA)
These features distinguish viruses from bacteria (which are living cells that reproduce independently and produce toxins) and fungi/protists (which are eukaryotic cells).
Worked Example 2: Explaining How a Vector Spreads Disease
Question: Malaria is spread by mosquitoes. Explain how the mosquito acts as a vector for malaria. [3 marks]
Solution:
- A female Anopheles mosquito feeds on the blood of a person infected with malaria and ingests the malaria protist (1 mark)
- The protist reproduces inside the mosquito without harming it — the mosquito is a vector, not a host that gets the disease (1 mark)
- When the same mosquito bites another person, it injects the protist into their bloodstream along with its saliva, infecting the new person (1 mark)
Worked Example 3: Suggesting How to Prevent the Spread of a Disease
Question: A new bacterial disease is spreading rapidly through a school. Suggest two methods that could be used to reduce the spread of this disease. Explain how each method works. [4 marks]
Solution:
Method 1 — Improved hygiene: Students and staff should wash hands regularly with soap and water and use hand sanitiser. This removes bacteria from hands before they can be transferred to surfaces or other people. Surfaces should be disinfected to kill any bacteria present. (2 marks)
Method 2 — Isolating infected individuals: Students who are infected should stay at home until they are no longer infectious. This prevents them from passing the bacteria to others through direct contact or airborne droplets. (2 marks)
Worked Example 4: Distinguishing Between Bacteria and Viruses
Question: A student says "Bacteria and viruses are both tiny so they must cause disease in the same way." Is the student correct? Explain your answer. [3 marks]
Solution:
The student is incorrect.
- Bacteria cause disease by producing toxins that damage tissues and make you feel ill. Bacteria are living cells that reproduce rapidly inside the body. (1 mark)
- Viruses cause disease by living and reproducing inside living cells, then bursting out and destroying the cells. Viruses are not living cells — they are just genetic material in a protein coat. (1 mark)
- Although both are small, their mechanisms of causing disease are fundamentally different. (1 mark)
Worked Example 5: Applying Knowledge to an Unfamiliar Scenario
Question: A new disease is spreading in a coastal town. People who eat raw oysters from the local bay are getting sick, but people who eat cooked oysters are fine. The disease is caused by a pathogen. Suggest what type of pathogen is likely to be responsible and explain why cooking the oysters prevents the disease. [4 marks]
Solution:
The pathogen is likely to be bacteria.
- Bacteria are commonly found in water and can contaminate seafood. (1 mark)
- Bacteria produce toxins that cause food poisoning symptoms. (1 mark)
- Cooking the oysters heats them to a high temperature, which kills the bacteria and denatures their toxins. (1 mark)
- Without live bacteria and active toxins, the cooked oysters do not cause disease. (1 mark)
Worked Example 6: Evaluating Methods of Controlling Malaria
Question: Malaria kills hundreds of thousands of people each year. Describe two ways of controlling the spread of malaria and explain a limitation of each. [4 marks]
Solution:
Method 1 — Using insecticide-treated mosquito nets: Sleeping under nets treated with insecticide prevents mosquitoes from biting people at night, which is when Anopheles mosquitoes feed. Limitation: Nets only protect people while they are in bed — people are still at risk of being bitten at other times or when outside. (2 marks)
Method 2 — Draining standing water / using insecticides: Removing pools of stagnant water removes the breeding grounds where mosquitoes lay their eggs, reducing the mosquito population. Insecticides kill adult mosquitoes. Limitation: Mosquitoes can develop resistance to insecticides over time, and draining all standing water across large areas can be impractical. (2 marks)
Practice Questions
Q1. What is a pathogen? Foundation [2 marks]
Q2. Name the four types of pathogen and give one example disease caused by each. Foundation [4 marks]
Q3. Describe how Salmonella is spread and explain two ways to prevent infection. Foundation Higher [4 marks]
Q4. Malaria is spread by mosquitoes. Explain how the mosquito acts as a vector for the malaria protist. Foundation Higher [3 marks]
Q5. Compare how bacteria and viruses cause disease. Include similarities and differences. Higher [4 marks]
Q6. A gardener notices black spots on the leaves of their roses. The leaves are turning yellow and falling off. (a) Name the disease and the type of pathogen that causes it. (b) Explain how the disease is spread. (c) Suggest two actions the gardener should take. Foundation Higher [5 marks]
Answers
A1. A pathogen is a microorganism (1 mark) that causes disease (1 mark).
A2.
- Bacteria — e.g. Salmonella / Gonorrhoea (1 mark)
- Viruses — e.g. HIV/AIDS / Measles / Tobacco mosaic virus (1 mark)
- Fungi — e.g. Rose black spot (1 mark)
- Protists — e.g. Malaria (1 mark)
A3. Salmonella is spread by eating food contaminated with Salmonella bacteria, such as undercooked poultry or unpasteurised milk (1 mark). Prevention method 1: Cook food thoroughly to kill any bacteria present (1 mark). Prevention method 2: Avoid cross-contamination by keeping raw and cooked foods separate and washing hands after handling raw meat (1 mark). Alternatively: refrigerate food properly / vaccinate poultry against Salmonella (1 mark).
A4. A mosquito bites a person infected with malaria and ingests the protist in their blood (1 mark). The protist does not cause disease in the mosquito — the mosquito carries it without being harmed (1 mark). When the mosquito bites another person, it injects the protist into their bloodstream, infecting them (1 mark).
A5.
Similarity: Both bacteria and viruses are microorganisms that cause communicable/infectious diseases (1 mark).
Differences:
- Bacteria are living cells that reproduce rapidly inside the body; viruses are not living cells and can only reproduce inside a host cell (1 mark)
- Bacteria cause disease by producing toxins that damage tissues; viruses cause disease by reproducing inside cells and destroying them when they burst out (1 mark)
- Bacteria can be killed by antibiotics; viruses cannot be treated with antibiotics (1 mark)
A6.
(a) Rose black spot, caused by a fungus (1 mark).
(b) The fungal spores are spread by wind and by water splashing from infected leaves onto healthy leaves (2 marks).
(c) Any two from: remove and destroy the infected leaves to prevent spores spreading (1 mark); spray the plant with fungicides to kill the fungus (1 mark); avoid overhead watering to reduce water splash; grow resistant varieties of roses.
Exam Tips
- Don't confuse bacteria and viruses. Bacteria are living cells; viruses are not. Bacteria produce toxins; viruses destroy cells from within.
- Learn the specific pathogen type for each disease. Exam questions often ask "What type of pathogen causes malaria?" — the answer is a protist, not a virus.
- Use the correct terminology. A vector carries the pathogen — the vector (mosquito) does not cause malaria. The protist causes malaria; the mosquito is just the vector.
- For "explain" questions, use a chain of reasoning. Don't just state a fact — explain the mechanism. E.g. "Cooking kills bacteria" is weaker than "Cooking heats food to a temperature high enough to kill bacteria and denature their toxins, preventing infection."
- For "compare" questions, you must give both similarities AND differences to get full marks. Making separate points about each is not enough — you need to explicitly compare.
- Viruses are NOT living cells. They are just genetic material inside a protein coat. They cannot reproduce on their own — they need a host cell.
- Salmonella symptoms are caused by toxins, not by the bacteria directly damaging tissue. This is a common distinction examiners test.
🔢 Maths Skills
Mathematical Skills
You need to interpret transmission data — this means reading bar charts showing disease rates in different countries, calculating how many people are infected from percentage data, and understanding exponential growth of pathogen populations.
Maths Example
In a population of 200,000, 0.15% are infected with a disease. How many people are infected?
Infected = 0.15 ÷ 100 × 200,000 = 0.0015 × 200,000 = 300 people
⚠️ Common Misconceptions
Watch Out!
1. Wrong: All bacteria cause disease Correct: Most bacteria are harmless and many are beneficial (e.g. gut bacteria for digestion) — only a small fraction are pathogenic
2. Wrong: Viruses are alive Correct: Viruses are not living cells — they cannot reproduce without a host cell, do not carry out metabolism, and consist only of genetic material in a protein coat
✍️ 6-Mark Question
Extended Answer
6 marks: Compare how bacteria and viruses cause disease.
Both bacteria and viruses are pathogens that cause communicable diseases. Bacteria are living, single-celled organisms that reproduce rapidly inside the body by binary fission. They cause disease by producing toxins (poisonous chemicals) that damage tissues and make you feel ill — for example, Salmonella produces toxins that cause vomiting and diarrhoea. Viruses are not living cells — they are genetic material inside a protein coat. They cause disease by entering living host cells, using the cell's machinery to replicate, and then bursting out of the cell, destroying it in the process. For example, HIV enters white blood cells and destroys them, weakening the immune system. A key difference is that bacteria cause harm through toxins while viruses cause harm through cell destruction. Another difference is that antibiotics can kill bacteria but cannot kill viruses because viruses hide inside body cells.
Mark scheme: 1 mark for similarity (both cause communicable disease); 1 mark for bacteria mechanism (toxins); 1 mark for bacteria example; 1 mark for virus mechanism (cell invasion and destruction); 1 mark for virus example; 1 mark for comparison of treatment implications
📊 AO3: Analyse & Evaluate
Analysis and Evaluation
A disease outbreak affects 500 people in a town. The data shows:
- 380 people who became ill had eaten at a local restaurant
- 120 people who became ill had not eaten at the restaurant
- 2000 people in the town ate at the restaurant and did not become ill
(a) What type of pathogen is most likely if symptoms include vomiting and diarrhoea 6 hours after eating?
(b) Evaluate whether the restaurant is the source of the outbreak.
Answers: (a) Bacteria (e.g. Salmonella) — bacterial food poisoning typically causes vomiting and diarrhoea within hours via toxin production. (b) While 380 of the 500 ill people ate at the restaurant, 120 did not, suggesting another source or secondary transmission. Also, 2000 people ate there without becoming ill, which may indicate that only certain dishes were contaminated. More investigation (e.g. testing food samples) is needed.
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