B12: Pathogens and Disease
Bacteria, viruses, fungi, protists, disease transmission, and specific diseases
Key Definitions
Pathogen โ A microorganism that causes disease (bacteria, virus, fungus, or protist).
Communicable disease โ A disease caused by pathogens that can be spread from one individual to another.
Vector โ An organism that carries and transmits a pathogen from one host to another (e.g. mosquitoes transmit malaria).
Toxin โ A poisonous substance produced by some pathogens that damages body tissues.
Communicable disease โ A disease caused by pathogens that can be spread from one individual to another.
Vector โ An organism that carries and transmits a pathogen from one host to another (e.g. mosquitoes transmit malaria).
Toxin โ A poisonous substance produced by some pathogens that damages body tissues.
Four Types of Pathogens
| Feature | Bacteria | Viruses | Fungi | Protists |
|---|---|---|---|---|
| Cell type | Prokaryotic (no nucleus) | Not cells โ just protein coat + genetic material | Eukaryotic | Eukaryotic (often single-celled) |
| Size | Very small (1โ5 ฮผm) | Extremely small (0.02โ0.3 ฮผm) | Varies (can be microscopic or visible) | Varies (mostly microscopic) |
| How they cause disease | Produce toxins that damage cells and tissues | Invade host cells and replicate inside them, bursting the cells | Grow on/inside organisms; can produce toxins; digest living tissue | Can be parasitic; damage tissues directly; some have complex life cycles |
| Can be treated with antibiotics? | Yes (but not viruses) | No | Some (antifungals) | Some (antiprotozoal drugs) |
How Pathogens Spread
Water โ Contaminated water supplies transmit waterborne diseases (e.g. cholera through faeces-contaminated water).
Air โ Airborne pathogens are carried in droplets produced by coughing/sneezing (e.g. influenza, measles, tuberculosis).
Direct contact โ Touching an infected person, contaminated surfaces, or bodily fluids (e.g. HIV, gonorrhoea, athlete's foot).
Vectors โ Animals that carry pathogens from one host to another (e.g. Anopheles mosquitoes transmit the malaria protist; fleas, ticks).
Air โ Airborne pathogens are carried in droplets produced by coughing/sneezing (e.g. influenza, measles, tuberculosis).
Direct contact โ Touching an infected person, contaminated surfaces, or bodily fluids (e.g. HIV, gonorrhoea, athlete's foot).
Vectors โ Animals that carry pathogens from one host to another (e.g. Anopheles mosquitoes transmit the malaria protist; fleas, ticks).
Specific Diseases
| Disease | Pathogen Type | Symptoms | Transmission | Prevention |
|---|---|---|---|---|
| Salmonella | Bacterium | Food poisoning: fever, abdominal cramps, vomiting, diarrhoea | Eating contaminated food (raw chicken, unpasteurised milk); poor food hygiene | Proper cooking; pasteurisation; hygiene in food preparation; keeping raw and cooked food separate |
| Gonorrhoea | Bacterium | Painful urination; discharge from penis/vagina; can cause infertility | Sexual contact (STI) | Use of condoms; limiting sexual partners; early treatment with antibiotics |
| HIV/AIDS | Virus | Flu-like illness initially; then may be asymptomatic for years; eventually immune system severely damaged leading to AIDS | Sexual contact; sharing needles; mother to child during birth or breastfeeding; infected blood | Use of condoms; needle exchange programmes; antiretroviral drugs (control but do not cure); screening of blood donations |
| Measles | Virus | Red rash, fever, cough; can be fatal; can cause blindness and brain damage | Airborne droplets (coughing/sneezing); highly contagious | MMR vaccination; isolation of infected individuals |
| Tobacco mosaic virus (TMV) | Virus | Mosaic pattern of discolouration on leaves; stunted growth; reduces photosynthesis and crop yield | Contact between infected and healthy plants; contaminated tools | Using disease-free seeds and plants; disinfecting tools; destroying infected plants; crop rotation |
| Rose black spot | Fungus | Purple/black spots on leaves; leaves turn yellow and drop early; reduces photosynthesis and growth | Spores carried by wind and water; spread by rain splashing; contaminated tools | Removing and burning affected leaves; using fungicides; ensuring good air circulation; avoiding overhead watering |
| Malaria | Protist | Recurring fever, chills, headache, vomiting; can be fatal; damages red blood cells and liver | Vector: female Anopheles mosquito carries protist; transmitted through mosquito bite | Mosquito nets (insecticide-treated); insect repellent; draining standing water; antimalarial drugs; indoor residual spraying with insecticide |
Example 1: How Malaria Is Transmitted
The malaria protist (Plasmodium) has a complex life cycle involving two hosts. When an infected female Anopheles mosquito bites a human, it injects the protist into the bloodstream. The protist travels to the liver, multiplies, then infects red blood cells, causing them to burst โ producing the characteristic recurring fever. When another mosquito bites the infected human, it ingests the protist and can transmit it to the next person.
Example 2: How TMV Reduces Crop Yield
TMV causes a mosaic pattern of discolouration on leaves because the virus destroys chloroplasts in the infected cells. This reduces the area of green leaf tissue available for photosynthesis, so the plant produces less glucose, grows less vigorously, and crop yields are reduced. Severely infected plants may also be stunted because less energy is available for growth.
Example 3: Preventing Salmonella Food Poisoning
Salmonella bacteria are commonly found on raw meat (especially poultry) and in unpasteurised milk. To prevent infection: cook meat thoroughly (to at least 70ยฐC) to kill bacteria; avoid unpasteurised milk and dairy products; keep raw meat separate from ready-to-eat food; wash hands after handling raw meat; store food at correct temperatures (below 5ยฐC or above 63ยฐC); in the UK, poultry is vaccinated against Salmonella.
Example 4: Why HIV Cannot Be Cured with Antibiotics
Antibiotics only kill bacteria by targeting their cell walls, protein synthesis machinery, or other prokaryotic features. HIV is a virus, not a bacterium โ it has no cell wall, no independent metabolism, and it replicates inside human host cells using the host's own machinery. Antibiotics have no target in a virus and cannot affect it. Antiretroviral drugs can control HIV replication but cannot eliminate the virus from the body entirely.
Example 5: Comparing Bacterial and Viral Disease Transmission
Both Salmonella (bacterial) and measles (viral) can be prevented by hygiene measures, but they spread differently. Salmonella spreads through contaminated food and requires ingestion, so prevention focuses on food safety. Measles spreads through airborne droplets from coughing and sneezing, so it is much more contagious and prevention focuses on vaccination and isolation of infected individuals. Both can cause serious illness but require different control strategies.
Exam tip: When describing a disease, always include: (1) the type of pathogen, (2) how it is transmitted, (3) symptoms, and (4) prevention. Do not confuse the pathogen type โ Salmonella and gonorrhoea are bacteria; measles, HIV, and TMV are viruses; rose black spot is a fungus; malaria is a protist.
Practice Questions
1. Foundation Name the four types of pathogen and give one example disease for each.
Bacteria โ Salmonella (food poisoning) or gonorrhoea. Viruses โ measles or HIV/AIDS or tobacco mosaic virus. Fungi โ rose black spot. Protists โ malaria.
2. Foundation Describe three ways that pathogens can be spread and give an example for each.
1) Water โ e.g. cholera bacteria spread through contaminated water. 2) Air โ e.g. measles virus spread through airborne droplets from coughing/sneezing. 3) Direct contact โ e.g. gonorrhoea bacteria spread through sexual contact. 4) Vectors โ e.g. malaria protist spread by Anopheles mosquito. (Any three.)
3. Higher Explain why antibiotics cannot be used to treat HIV.
Antibiotics target prokaryotic features such as bacterial cell walls, ribosomes, and DNA replication machinery. HIV is a virus, not a bacterium โ it has no cell wall and no independent metabolism. Viruses replicate inside human host cells using the host's own cellular machinery. Antibiotics have no target in a virus and therefore cannot affect it.
4. Higher Describe how rose black spot is transmitted and how it can be controlled.
Rose black spot is caused by a fungus. It is transmitted when spores are carried by wind and water (rain splashing) from infected leaves to healthy plants, or via contaminated gardening tools. Control methods include: removing and burning affected leaves to prevent spore spread, using fungicide sprays, ensuring good air circulation between plants to reduce humidity, avoiding overhead watering that splashes spores, and disinfecting tools after use.
5. Foundation Explain how malaria is transmitted and describe two methods of prevention.
Malaria is caused by a protist (Plasmodium) and transmitted when an infected female Anopheles mosquito bites a human, injecting the protist into the bloodstream. Prevention: 1) Sleep under insecticide-treated mosquito nets to prevent mosquito bites. 2) Use insect repellent on skin. 3) Drain standing water where mosquitoes breed. 4) Take antimalarial drugs before travelling to endemic areas. 5) Spray insecticide indoors. (Any two.)
6. Higher Explain how tobacco mosaic virus reduces crop yield.
TMV causes a mosaic pattern of discolouration on leaves by destroying chloroplasts in infected cells. This reduces the green leaf area available for photosynthesis, so the plant produces less glucose. With less energy available, plant growth is stunted and crop yield is reduced. Severely infected plants may also die prematurely.
๐ข Maths Skills
Mathematical Skills
Interpreting disease transmission data: read and interpret graphs showing how disease spreads through a population over time. For example, an epidemic curve may show the number of new cases per day โ calculate the total number of cases, identify the peak day, and determine whether the disease is spreading (increasing cases) or being controlled (decreasing cases). You may also need to calculate the rate of spread: number of new cases รท time period.
โ ๏ธ Common Misconceptions
Watch Out!
Students often think all bacteria cause disease. Wrong: All bacteria are harmful and cause illness Correct: Most bacteria are harmless and many are beneficial โ only a small fraction are pathogenic and produce toxins that cause disease
Students often think viruses are alive. Wrong: Viruses are living organisms Correct: Viruses are not alive โ they have no cellular structure, no metabolism, and cannot reproduce without a host cell
โ๏ธ 6-Mark Question
Extended Answer
6 marks: Compare how bacteria and viruses cause disease and spread.
Bacteria are living prokaryotic cells that cause disease by producing toxins that damage cells and tissues โ for example, Salmonella produces toxins that cause food poisoning symptoms (vomiting, diarrhoea). Bacteria can reproduce rapidly by binary fission outside the body in suitable conditions. Viruses are not cells โ they consist of genetic material inside a protein coat. They cause disease by invading host cells and using the cell's machinery to replicate, then bursting the cell, killing it โ for example, HIV invades white blood cells and destroys them, weakening the immune system. Both can spread through similar routes: direct contact (gonorrhoea bacteria, HIV virus), airborne droplets (tuberculosis bacteria, measles virus), and contaminated food/water (Salmonella bacteria, norovirus). However, viruses cannot be treated with antibiotics, whereas bacterial infections can. Prevention strategies overlap (hygiene, vaccination) but differ in specifics โ bacterial infections may be treated with antibiotics, while viral infections rely on prevention and the immune system.
Mark scheme: 1 mark for explaining how bacteria cause disease (toxins); 1 mark for explaining how viruses cause disease (invade and burst cells); 1 mark for comparing reproduction; 1 mark for comparing transmission routes with named examples; 1 mark for comparing treatment (antibiotics vs no cure); 1 mark for a clear comparison structure (similarities and differences)
๐ AO3: Analyse & Evaluate
Analysis and Evaluation
Given pathogen data, identify the type of pathogen and suggest control measures. For example: a disease spreads rapidly through a school, causing vomiting and diarrhoea within 24 hours of exposure. It is transmitted via contaminated food in the canteen. The pathogen reproduces inside gut cells and is unaffected by antibiotics. This is likely a virus (rapid spread, unaffected by antibiotics, replicates inside cells). Control measures: isolate affected individuals, close the canteen for deep cleaning, promote handwashing, and ensure food is prepared hygienically. If the pathogen were bacterial (e.g. Salmonella), antibiotics could be used for severe cases and the focus would be on thorough cooking and preventing cross-contamination.
๐ Exam Questions by Topic
๐ฌ Video Resources
Ready to ace your GCSE Biology exams?
Get the best revision books and guides to boost your grades.