Pathogens and How They Spread
A pathogen is a microorganism that causes disease. In this lesson you will learn about the four types of pathogen, how they cause disease, how they spread from one organism to another, and how we can reduce the spread of infection.
What Is a Pathogen?
A pathogen is a microorganism that enters the body and causes disease. Pathogens include:
- Bacteria
- Viruses
- Fungi
- Protists
Not all microorganisms are pathogens. Many bacteria are harmless or even beneficial (e.g. gut bacteria that help with digestion). Only those that cause disease are called pathogens.
The Four Types of Pathogen
1. Bacteria
- Single-celled prokaryotic organisms (no nucleus)
- Reproduce rapidly by binary fission (one cell splits into two)
- Cause disease by releasing toxins — poisonous chemicals that damage cells and tissues
- Some bacteria are beneficial; only a small proportion are pathogenic
- Size: typically 1–5 μm
Examples: Salmonella, Neisseria gonorrhoeae, Mycobacterium tuberculosis
2. Viruses
- Not true living organisms — they are particles, not cells
- Much smaller than bacteria (typically 20–300 nm)
- Cannot reproduce on their own — they invade host cells and use the cell's machinery to make copies of themselves
- The host cell eventually bursts (lysis), releasing new virus particles that infect more cells
- This process of cell destruction is what causes the symptoms of viral disease
Examples: HIV, measles virus, tobacco mosaic virus (TMV), influenza virus
3. Fungi
- Can be single-celled (yeasts) or multicellular (moulds and mushrooms)
- Some fungi send out hyphae (thread-like structures) that grow into tissue and digest it
- Can produce spores that spread through air, water, or direct contact
- Some fungi release toxins or cause allergic reactions
Examples: Rose black spot (Diplocarpon rosae), athlete's foot (Trichophyton)
4. Protists
- Eukaryotic single-celled organisms (they have a nucleus)
- Some are parasites that rely on a vector to transfer them between hosts
- A vector is an organism that carries a pathogen from one host to another without being affected by the disease itself
Examples: Plasmodium (causes malaria, spread by the Anopheles mosquito vector)
Comparison of the Four Pathogen Types
| Feature | Bacteria | Viruses | Fungi | Protists |
|---|
| Cell type | Prokaryotic | Not cells (particles) | Eukaryotic | Eukaryotic |
| Size | 1–5 μm | 20–300 nm | Variable | Variable |
| Reproduction | Binary fission | Inside host cells | Spores / budding | Binary fission |
| How they cause disease | Release toxins | Cell lysis (bursting) | Hyphae / toxins | Tissue damage |
| Living? | Yes | Debatable | Yes | Yes |
| Example disease | Salmonella | Measles | Rose black spot | Malaria |
Exam tip: You must know the difference between how bacteria and viruses cause disease. Bacteria produce toxins that damage cells; viruses invade cells, replicate, and cause lysis (bursting) of the host cell. This is a very common exam question.
How Pathogens Are Spread
Pathogens can be transmitted in several ways:
1. Direct Contact
- Person to person through touching, kissing, or sexual contact
- Examples: gonorrhoea (sexual contact), athlete's foot (skin contact)
- Also includes contact with contaminated surfaces (fomites)
2. Airborne (Droplet Infection)
- When an infected person coughs, sneezes, or talks, tiny droplets containing pathogens are released into the air
- Other people breathe in these droplets
- Examples: measles, TB, influenza, COVID-19
3. Water Contamination
- Drinking or swimming in water contaminated with sewage or animal waste
- Pathogens in the water enter the body through the mouth or through wounds
- Examples: cholera (Vibrio cholerae), dysentery
4. Vectors
- A vector is an organism that carries a pathogen between hosts without being affected itself
- The most important vector for GCSE Biology is the Anopheles mosquito, which carries the Plasmodium protist that causes malaria
- The mosquito feeds on blood from an infected person, takes up the Plasmodium, then injects it into the next person it bites
- Other vectors: ticks (Lyme disease), houseflies (food contamination)
5. Contaminated Food
- Pathogens can be present in undercooked or improperly stored food
- Examples: Salmonella in undercooked poultry or eggs
| Method of spread | How it works | Example disease |
|---|
| Direct contact | Touching, kissing, sexual contact | Gonorrhoea, athlete's foot |
| Airborne droplets | Coughing/sneezing releases pathogen-laden droplets | Measles, TB, flu |
| Water | Contaminated drinking/bathing water | Cholera |
| Vectors | Organism carries pathogen between hosts | Malaria (mosquito) |
| Food | Contaminated or undercooked food | Salmonella |
Preventing the Spread of Pathogens
Personal Hygiene
- Handwashing with soap and water removes pathogens from skin
- Covering mouth and nose when coughing or sneezing reduces airborne spread
- Using tissues and disposing of them immediately
- Not sharing personal items (towels, toothbrushes)
Public Health Measures
- Quarantine and isolation of infected individuals to prevent spread
- Clean water supplies — treating water to kill pathogens prevents waterborne diseases
- Sewage treatment — prevents contamination of drinking water
- Vaccination programmes — reduce the number of susceptible individuals in a population (more on this in a later lesson)
Vector Control
- Mosquito nets — prevent mosquitoes biting people while they sleep
- Insecticides — kill mosquitoes
- Draining standing water — removes mosquito breeding sites (mosquitoes lay eggs in still water)
- Biological control — introducing predators or sterile male mosquitoes
Food Safety
- Proper cooking — ensures internal temperature is high enough to kill pathogens
- Correct storage — refrigeration slows bacterial growth
- Avoiding cross-contamination — keeping raw and cooked foods separate
- Vaccination of livestock — e.g. chickens vaccinated against Salmonella
Exam tip: Questions about preventing disease spread are very common. Match the prevention method to the mode of transmission. For example, mosquito nets prevent malaria (vector-borne), handwashing prevents foodborne illness (food contamination), and isolation prevents airborne diseases.