Communicable Diseases — Pathogens and Transmission

This lesson introduces communicable (infectious) diseases, the four types of pathogen, and how they are transmitted between organisms. This is the foundation of the Infection and Response topic in the AQA GCSE Combined Science Trilogy specification (8464).

What Is a Communicable Disease?

A communicable disease (also called an infectious disease) is one that can be spread from one organism to another. These diseases are caused by pathogens — microorganisms that enter the body and cause illness.

It is important to distinguish communicable diseases from non-communicable diseases:

Term	Definition	Examples
Communicable disease	Caused by a pathogen; can spread between organisms	Measles, HIV, salmonella
Non-communicable disease	Not caused by a pathogen; cannot be passed on	Cancer, heart disease, diabetes

Exam Tip: The specification uses the term "communicable" rather than "infectious" or "contagious". Always use "communicable disease" in your answers.

The Four Types of Pathogen

AQA requires you to know four types of pathogen:

Pathogen Type	Size	Living?	How It Causes Disease	Examples
Bacteria	~1–5 $\mu m$	Yes — prokaryotic cells	Reproduce rapidly by binary fission; release toxins that damage cells and tissues	Salmonella, gonorrhoea
Viruses	~20–300 nm	Debated — not true cells	Invade host cells, hijack cell machinery to replicate, then the host cell bursts (lysis), releasing new viruses	HIV, measles, TMV
Fungi	Variable	Yes — eukaryotic	Hyphae grow into tissues; may produce spores that spread	Athlete's foot, rose black spot
Protists	~10–100 $\mu m$	Yes — eukaryotic	Often parasites; frequently spread by vectors	Malaria (Plasmodium)

How Do Pathogens Make Us Ill?

Pathogens cause disease in two main ways:

Cell damage — viruses destroy host cells when they burst out; bacteria and fungi damage tissues as they grow and multiply.
Toxin production — many bacteria release toxins (poisonous chemicals) that damage surrounding cells and make us feel ill.

The symptoms we experience (e.g. fever, inflammation, pain) are often a combination of damage caused by the pathogen and the body's own immune response.

Exam Tip: Never say pathogens cause disease "on purpose" — they are simply reproducing. The damage is a side effect of their life cycle.

How Pathogens Are Transmitted

Pathogens spread between organisms in several ways:

Method of Transmission	Description	Example Diseases
Airborne (droplet infection)	Inhaling tiny droplets released when an infected person coughs, sneezes or breathes	Measles, influenza
Direct contact	Touching an infected person, contaminated surface, or exchange of body fluids	Gonorrhoea (sexual contact), athlete's foot
Waterborne	Drinking or contact with contaminated water	Cholera
Vector transmission	Carried by another organism (the vector) which transfers the pathogen without being affected itself	Malaria (spread by mosquitoes)
Contaminated food	Eating food containing pathogens due to poor hygiene or undercooked preparation	Salmonella

graph LR
    S[Infected Source] --> A[Airborne droplets]
    S --> B[Direct contact]
    S --> C[Contaminated water]
    S --> D[Vector - e.g. mosquito]
    S --> E[Contaminated food]
    A --> H[New Host]
    B --> H
    C --> H
    D --> H
    E --> H

Conditions for Bacterial Growth

Bacteria reproduce by binary fission — splitting into two identical daughter cells. Under ideal conditions this can happen every 20 minutes.

$$N = N_0 \times 2^n$$

Where:

$N$ = final number of bacteria
$N_0$ = initial number of bacteria
$n$ = number of divisions (generations)

For example, starting with 1 bacterium dividing every 20 minutes for 6 hours (18 divisions):

$$N = 1 \times 2^{18} = 262{,}144 \text{ bacteria}$$

This rapid growth explains why infections can develop extremely quickly.

Factor	Optimum Condition
Temperature	~37 °C (body temperature) for human pathogens
Nutrients	Glucose and other organic molecules
Moisture	Water is essential for chemical reactions
pH	Most bacteria prefer neutral pH (~7)

Preventing the Spread of Disease

Because pathogens spread in predictable ways, we can take steps to reduce transmission:

Prevention Method	How It Works
Hand washing	Removes pathogens from skin before transfer
Vaccination	Stimulates immunity before infection occurs
Isolation / quarantine	Separates infected individuals to stop spread
Safe food preparation	Cooking food thoroughly kills bacteria
Clean water supply	Treating water removes waterborne pathogens
Insect nets / insecticides	Prevents contact with vectors such as mosquitoes
Using condoms	Prevents sexually transmitted infections (STIs)

Common Mistakes

Mistake	Correction
"Viruses are living organisms"	Viruses are not considered living — they cannot reproduce without a host cell
"Antibiotics kill viruses"	Antibiotics only work against bacteria; they have no effect on viruses
"All bacteria are harmful"	The vast majority of bacteria are harmless or beneficial; only a small number are pathogenic
"Pathogens are the same as antigens"	Pathogens are the organisms; antigens are molecules on their surface that trigger an immune response

Summary

Communicable diseases are caused by pathogens and can spread between organisms
The four types of pathogen are bacteria, viruses, fungi, and protists
Bacteria produce toxins; viruses invade and destroy host cells by lysis
Pathogens spread via airborne droplets, direct contact, water, vectors, and contaminated food
Bacteria reproduce by binary fission and can multiply extremely rapidly under ideal conditions
Prevention includes hygiene, vaccination, isolation, food safety, clean water, and vector control

Exam Tip: When asked to describe how a named pathogen causes disease, always include: (1) how the pathogen enters the body, (2) how it reproduces, and (3) what damage it causes (toxins or cell destruction). This three-part structure will help you gain full marks.