Pathogens and the spread of infection

Q: What are the pathogens and transmission methods for Cholera and Tuberculosis?

Cholera is caused by the bacterium Vibrio cholerae spread via contaminated water; Tuberculosis is caused by the bacterium Mycobacterium tuberculosis spread through the air.

Q: Which pathogens cause stomach ulcers and haemorrhagic fever?

Stomach ulcers are caused by the bacterium Helicobacter pylori (oral transmission); haemorrhagic fever is caused by the Ebola virus (body fluids).

Biology (1BI0) · Pearson Edexcel GCSE (9-1) · 5 min read · Updated: 7 Sept 2026

Introduction to Pathogens

In this chapter, we are going to look at the "uninvited guests" of the biological world: pathogens. A pathogen is simply a microorganism that causes disease. When these pathogens jump from one person to another, we call the result a communicable disease (an infectious disease).

Don't worry if the names of the diseases sound complicated at first. We will break them down into four main "types" of pathogens and look at exactly how they spread and, more importantly, how we can stop them!

The Four Types of Pathogen

There are four main categories of pathogens you need to know for your Edexcel exam:

1. Bacteria: Very small cells that can reproduce rapidly inside your body. They can make you feel ill by producing toxins (poisons) that damage your cells and tissues.
2. Viruses: These are not cells. They are much smaller and live inside your cells, using the cell's "machinery" to produce copies of themselves. Eventually, the cell bursts, releasing the new viruses.
3. Protists: These are eukaryotic organisms. Most are single-celled. Some are parasites that live on or inside other organisms.
4. Fungi: Some are single-celled, while others have a body made up of thread-like structures called hyphae. They can produce spores which spread the infection.

Quick Tip: Think of a pathogen as a "tiny criminal" and the way it spreads as its "getaway method." To stop the crime (the disease), we need to block the getaway route!

Common Pathogens and How They Spread

The syllabus requires you to know specific examples of diseases. Use the table below to help you remember the pathogen type and its transmission method.

1. Cholera

Pathogen: A bacterium called Vibrio cholerae.
Symptoms: Severe watery diarrhoea.
How it spreads: Through contaminated water sources.
Reducing the spread: Making sure people have access to clean water supplies and improved sanitation (proper sewage systems).

2. Tuberculosis (TB)

Pathogen: A bacterium called Mycobacterium tuberculosis.
Symptoms: Coughing and lung damage.
How it spreads: It is airborne. When an infected person coughs or sneezes, the bacteria spread in tiny droplets in the air.
Reducing the spread: Infected people should avoid crowded public spaces, practice good hygiene (covering the mouth), and ensure sleep areas are well-ventilated.

3. Chalara Ash Dieback

Pathogen: A fungus that infects ash trees.
Symptoms: Leaf loss and bark lesions (wounds).
How it spreads: Carried through the air by the wind. It can also spread when diseased ash trees are moved between different areas.
Reducing the spread: Removing infected ash trees and replanting with different species, or restricting the movement of ash trees.

4. Malaria

Pathogen: A protist.
Symptoms: Damage to red blood cells and the liver; repeating episodes of severe fever.
How it spreads: Via a vector. A vector is an animal that spreads disease without getting sick itself. In this case, it is the mosquito.
Reducing the spread: Use mosquito nets to prevent bites and apply insect repellent to kill the vectors.

5. HIV (Human Immunodeficiency Virus)

Pathogen: A virus.
Symptoms: It destroys white blood cells, which are part of the immune system. This eventually leads to AIDS, where the body's immune system fails and cannot fight off other infections.
How it spreads: Through body fluids (blood, semen, vaginal fluids). Common routes include unprotected sex and sharing needles.
Reducing the spread: Using condoms during sex and encouraging drug users not to share needles.

Biology Only: Helicobacter and Ebola

Note: If you are studying for the GCSE Biology (1BI0) qualification, you must also know these two specific diseases.

6. Helicobacter (Stomach Ulcers)

Pathogen: A bacterium called Helicobacter pylori.
Symptoms: Stomach pain, nausea, and vomiting. It causes ulcers (sores) in the stomach lining.
How it spreads: Usually through oral transmission (e.g., eating contaminated food or sharing cups).
Reducing the spread: Having clean water supplies and practicing good hygiene when handling food.

7. Ebola

Pathogen: Ebola virus.
Symptoms: Haemorrhagic fever (fever followed by severe internal bleeding).
How it spreads: Through body fluids (blood, vomit, faeces). It is highly contagious.
Reducing the spread: Isolating infected individuals and sterilising any areas where the virus might be present.

Summary Table for Quick Revision

Sexually Transmitted Infections (STIs)

STIs are infections spread through sexual contact. You need to know about two specific examples:

HIV

As mentioned above, HIV is a virus spread through body fluids. It is considered an STI because it is frequently passed on through unprotected sexual intercourse. It attacks the immune system.

Chlamydia

Pathogen: A bacterium.
Symptoms: Often has no symptoms, but it can cause infertility in both men and women if left untreated.
How it spreads: Sexual contact.
Reducing the spread: The best way to reduce spread is by using a barrier method of contraception, such as a condom. Screening (testing) individuals is also important so they can be treated with antibiotics and stop the spread to others.

How to Reduce the Spread of Infection (General)

If you are asked an "Evaluate" or "Explain" question about reducing infection, remember these four general strategies:

1. Hygiene: Washing hands and cleaning surfaces kills pathogens.
2. Reducing Vectors: Getting rid of the organisms that "carry" the disease (like mosquitoes or houseflies).
3. Quarantine/Isolation: Keeping infected people away from healthy people prevents the pathogen from jumping to a new host.
4. Vaccination: Protecting people so they cannot catch the disease in the first place (we will cover this in the "Immunisation" chapter!).

Common Mistakes to Avoid

Confusing Bacteria and Viruses: Remember, antibiotics only kill bacteria. They do nothing against viruses like HIV or Ebola.
Forgetting the Vector: In Malaria, the mosquito isn't the pathogen; it's the vector. The pathogen is the protist living inside the mosquito.
Chalara Spread: Students often think Chalara is spread by animals. It is actually spread by the wind and the movement of plants by humans.

Key Takeaways

Pathogens are microorganisms that cause disease. They include bacteria, viruses, protists, and fungi.
Diseases can be spread by water (Cholera), air (TB, Chalara), vectors (Malaria), body fluids (HIV, Ebola), or oral intake (Helicobacter).
The spread of STIs like HIV and Chlamydia is best reduced using barrier contraception (condoms).

Quick check

Can you answer these now?

Open each question to check the key ideas from this chapter.

What is a pathogen and what type of disease does it cause?

A pathogen is a microorganism that causes disease; these diseases are known as communicable (infectious) diseases.

How do bacteria cause illness in the body?

Bacteria are small cells that reproduce rapidly and produce toxins (poisons) that damage cells and tissues.

How do viruses replicate and cause damage to host cells?

Viruses live inside host cells and use the cell's machinery to replicate; they cause damage when the cell eventually bursts to release the new viruses.

What are the pathogens and transmission methods for Cholera and Tuberculosis?

Cholera is caused by the bacterium Vibrio cholerae spread via contaminated water; Tuberculosis is caused by the bacterium Mycobacterium tuberculosis spread through the air.

What is Chalara Ash Dieback and how is it transmitted?

It is a fungal infection of ash trees that causes leaf loss and bark lesions; it is spread through the air by wind or the movement of diseased trees.

What is a vector, and which disease uses a mosquito as one?

A vector is an animal that spreads a pathogen without getting sick itself; Malaria is spread by the mosquito vector.

How does HIV affect the body and how is it spread?

HIV is a virus that destroys white blood cells (weakening the immune system) and is spread through body fluids like blood or semen.

Which pathogens cause stomach ulcers and haemorrhagic fever?

Stomach ulcers are caused by the bacterium Helicobacter pylori (oral transmission); haemorrhagic fever is caused by the Ebola virus (body fluids).

How can the spread of Chlamydia be reduced?

The spread of Chlamydia (a bacterial STI) can be reduced through the use of barrier contraception like condoms and through regular screening.

What are the four general strategies for reducing the spread of infection?

1. Hygiene, 2. Reducing vectors, 3. Quarantine/Isolation, and 4. Vaccination.

Why are antibiotics ineffective against diseases like HIV or Ebola?

Antibiotics only kill bacteria; they have no effect on viruses.

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