# Pathogens and transmission routes

> CIE A-Level Biology · Unit 10: Infectious Disease
> Source: https://www.owlsprep.com/study/cie-9700-u10-pathogens-and-transmission-routes/

This sub-topic covers the definition of a pathogen, the main groups of disease-causing infectious agents, and the different routes pathogens use to spread between susceptible hosts, with named examples aligned to the CIE 9700 syllabus.

**Prerequisites:** [Basic cell structure of prokaryotes and eukaryotes](https://www.owlsprep.com/study/cie-9700-u1-cell-structure/)

## Learning objectives

- Distinguish between pathogens and non-pathogenic microorganisms
- Name and describe the main types of disease-causing pathogens
- Classify and describe different transmission routes of pathogens
- Give named examples of diseases for each pathogen and transmission type

## What is a Pathogen?

Many microorganisms live on or in the human body without causing harm; these are called commensals, or normal gut/ skin flora. A pathogen is any agent that causes infectious disease, which can be transmitted between hosts.

**Pathogen** — An infectious agent that causes disease by damaging host tissues and disrupting normal host physiological function

*Example:* Mycobacterium tuberculosis, which causes tuberculosis (TB)

Virulence describes a pathogen's ability to cause severe disease. Some commensal microorganisms can become opportunistic pathogens if they enter a new body site, or if the host's immune system is weakened.

**Worked example:** Explain why *Staphylococcus aureus*, found on the skin of many healthy people, can sometimes be classified as a pathogen.

1. 1. Recall that the core requirement for classification as a pathogen is that it causes disease.
2. 2. *S. aureus* is a harmless commensal on intact healthy skin, so it is not pathogenic here.
3. 3. If the skin is broken (e.g. via a cut or surgery), *S. aureus* can enter deep tissues or the bloodstream.
4. 4. It multiplies and damages host tissue, causing conditions like abscesses or sepsis.
5. 5. When it causes disease, it acts as a pathogen, even though it is harmless in its normal location.

> **Exam tip:** CIE often asks you to apply the definition of a pathogen to examples, so always reference the requirement to cause disease.

## Main Groups of Pathogens

CIE 9700 requires you to know five main groups of pathogens, with named examples for each group:

- **Bacteria**: Prokaryotic cells, e.g. *Vibrio cholerae* (cholera), *Mycobacterium tuberculosis* (TB)
- **Viruses**: Non-cellular infectious agents, e.g. Human Immunodeficiency Virus (HIV), influenza virus
- **Protoctista**: Eukaryotic parasites, e.g. *Plasmodium falciparum* (malaria)
- **Fungi**: Eukaryotic heterotrophs, e.g. *Trichophyton* (athlete's foot)
- **Parasitic worms**: Multicellular eukaryotic parasites, e.g. *Schistosoma* (schistosomiasis)

**Worked example:** Classify each of the following into the correct pathogen group: (a) *Plasmodium*, (b) HIV, (c) *Mycobacterium tuberculosis*

1. (a) *Plasmodium* is a single-celled eukaryotic parasite. It is classified as a protoctistan pathogen.
2. (b) HIV is a non-cellular infectious agent that only replicates inside host cells. It is classified as a virus.
3. (c) *Mycobacterium tuberculosis* is a single-celled prokaryotic organism. It is classified as a bacterium.

> **Exam tip:** Always use correct binomial naming (capital genus, lowercase species) for examples in written answers to get full marks.

## Transmission Routes

Transmission routes are split into two broad categories: direct (no intermediate between infected and new host) and indirect (involves an intermediate object or organism).

**Direct transmission** — Transmission that occurs through direct interaction between an infected host and a new susceptible host, no intermediate involved

*Example:* HIV transmission via unprotected sexual contact

**Indirect transmission** — Transmission that occurs via an intermediate: contaminated object (fomite), food/water, or vector organism

*Example:* Malaria transmission via *Anopheles* mosquitoes

| Transmission route | Type | Named disease example |
| --- | --- | --- |
| Direct contact | Direct | HIV, athlete's foot |
| Droplet inhalation | Direct | Influenza, TB |
| Faecal-oral | Indirect | Cholera, typhoid |
| Vector-borne | Indirect | Malaria |
| Airborne spores | Indirect | Anthrax |
| Wound entry | Mixed | Tetanus |

**Worked example:** Describe the transmission route of cholera and classify it as direct or indirect.

1. 1. Cholera is caused by the bacterium *Vibrio cholerae*, which lives in the gut of infected people.
2. 2. Bacteria are released from infected hosts in faeces.
3. 3. Bacteria contaminate water supplies or uncooked food, which is then ingested by a new susceptible host.
4. 4. This route is called faecal-oral transmission. It is indirect, because it uses contaminated water/ food as an intermediate between hosts.

## Factors Affecting Transmission

For a pathogen to successfully spread and cause an outbreak, three requirements must be met: a reservoir of the pathogen, a susceptible host population, and a viable transmission route between the reservoir and new hosts.

> **info**
>
> The chain of infection framework simplifies this: Reservoir → Portal of exit → Transmission route → Portal of entry → Susceptible host

Human and environmental factors modify transmission risk: overcrowding increases droplet transmission of respiratory pathogens, while lack of clean water increases faecal-oral transmission of enteric pathogens like cholera.

**Worked example:** Explain why malaria is only endemic in warm, wet tropical regions.

1. 1. Malaria is caused by the protoctistan pathogen *Plasmodium*, which is transmitted by female *Anopheles* mosquitoes (the vector).
2. 2. *Anopheles* mosquitoes require standing water to breed and complete their life cycle, and warm temperatures for development.
3. 3. Warm, wet tropical regions provide the ideal habitat for the vector population to persist and multiply.
4. 4. Without the vector, *Plasmodium* cannot be transmitted between humans, so malaria cannot establish endemicity in cooler or drier regions.

> **Exam tip:** Always link the transmission factor to the specific pathogen/ vector/ route to gain full marks in extended answer questions.

## Common pitfalls

- **Wrong:** Calling all microorganisms pathogens
  - Why it fails: Most microorganisms do not cause disease; only disease-causing agents are classified as pathogens
  - Correct: Always remember: the defining feature of a pathogen is that it causes disease
- **Wrong:** Claiming viruses are not pathogens because they are non-living
  - Why it fails: CIE 9700 explicitly lists viruses as pathogens regardless of their classification as living/ non-living
  - Correct: Accept viruses as a valid group of pathogens in 9700 exams
- **Wrong:** Classifying droplet infection as indirect transmission
  - Why it fails: Droplet infection is direct because it involves direct transfer from infected person to new host via inhaled droplets, no intermediate
  - Correct: Classify droplet infection as a type of direct transmission
- **Wrong:** Calling the mosquito the pathogen that causes malaria
  - Why it fails: The mosquito is the vector (carrier), not the pathogen that causes disease
  - Correct: Separate pathogen (*Plasmodium*, causes disease) from vector (mosquito, carries the pathogen)

## Cheatsheet

| Pathogen Group | Example | Transmission Route |
| --- | --- | --- |
| Bacterium | Mycobacterium tuberculosis | Droplet |
| Bacterium | Vibrio cholerae | Faecal-oral |
| Virus | HIV | Direct (sexual/blood) |
| Virus | Influenza | Droplet |
| Protoctist | Plasmodium falciparum | Vector-borne (Anopheles) |
| Fungus | Trichophyton | Direct contact |
| Parasitic worm | Schistosoma | Indirect (water contact) |

## What's next

This sub-topic forms the foundation for all other content in the infectious disease unit. Understanding the definition of pathogens and their transmission routes is critical for analysing global patterns of disease, the impact of major diseases like malaria, HIV and TB on human populations, and evaluating the effectiveness of control and prevention measures. Mastery of core definitions here will help you access marks for almost every question in this unit on exam day.

- [HIV/AIDS](https://www.owlsprep.com/study/cie-9700-u10-hiv-aids/)
- [Tuberculosis](https://www.owlsprep.com/study/cie-9700-u10-tuberculosis/)
- [Malaria](https://www.owlsprep.com/study/cie-9700-u10-malaria/)

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