Communicable Diseases and Pathogens

This lesson introduces the concept of communicable (infectious) diseases and the four types of pathogen that cause them. Understanding how diseases spread and how the body prevents entry of pathogens is fundamental to the entire Infection and Response topic in AQA GCSE Biology.

What Is a Communicable Disease?

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

Not all microorganisms are harmful. In fact, the vast majority of bacteria, fungi and protists are harmless or even beneficial. However, the small number that are pathogenic can cause serious disease.

Term	Definition
Communicable disease	A disease caused by a pathogen that can spread between organisms
Non-communicable disease	A disease that cannot be passed between organisms (e.g. cancer, heart disease)
Pathogen	A microorganism that causes disease
Infection	The invasion and multiplication of pathogens within the body
Incubation period	The time between infection and the appearance of symptoms

Exam Tip: Do not say pathogens are "bugs" or "germs" — always use the correct scientific term. The examiners want you to say pathogen and to name the specific type (virus, bacterium, fungus, protist).

The Four Types of Pathogen

There are four main types of pathogen you need to know for AQA GCSE Biology:

Pathogen Type	Size	Living?	How It Causes Disease	Examples
Bacteria	~1-5 micrometres	Yes — living cells	Reproduce rapidly inside the body; produce toxins that damage cells and tissues	Salmonella, E. coli, tuberculosis (TB)
Viruses	~20-300 nanometres	Debated — not true living cells	Enter host cells, hijack the cell machinery to replicate, then burst the cell (lysis) to release new viruses	Influenza, HIV, measles, TMV
Fungi	Variable	Yes — living organisms	Some are single-celled; others produce hyphae that grow into tissues; may produce spores	Athlete's foot, rose black spot
Protists	~10-100 micrometres	Yes — eukaryotic organisms	Often parasites; some are carried by vectors (organisms that spread disease without being affected)	Malaria (Plasmodium), dysentery

Bacteria vs Viruses — Key Differences

This is one of the most commonly examined comparisons in GCSE Biology:

Feature	Bacteria	Viruses
Size	Typically 1-5 micrometres	Much smaller: 20-300 nanometres
Cell structure	Prokaryotic cell (no nucleus)	Not a cell — just DNA/RNA in a protein coat
Reproduction	Binary fission (cell division)	Replicate inside host cells only
How they cause illness	Produce toxins	Damage/destroy host cells
Treated by antibiotics?	Yes	No — antibiotics do not work on viruses

Exam Tip: A classic exam question is: "Explain why antibiotics cannot treat viral diseases." The answer is that viruses live inside host cells and reproduce using the host cell's machinery, so antibiotics cannot target them without destroying the body's own cells.

How Pathogens Spread

Pathogens can spread between organisms in several ways:

Method of Transmission	Description	Example
Direct contact	Touching an infected person or contaminated surface	Athlete's foot from shared changing room floors
Airborne (droplet)	Inhaling tiny droplets released when an infected person coughs or sneezes	Influenza, measles, tuberculosis
Waterborne	Drinking or coming into contact with contaminated water	Cholera, dysentery
Vector transmission	Carried by another organism (the vector) which transfers the pathogen	Malaria spread by mosquitoes
Body fluids	Contact with blood, saliva or other fluids	HIV through blood or sexual contact
Contaminated food	Eating undercooked or improperly stored food containing pathogens	Salmonella from raw chicken

The following diagram shows the main routes of pathogen transmission:

graph LR
    P[Pathogen Source] --> A[Airborne droplets]
    P --> B[Direct contact]
    P --> C[Contaminated water]
    P --> D[Vector organism]
    P --> E[Body fluids]
    P --> F[Contaminated food]
    A --> H[New Host]
    B --> H
    C --> H
    D --> H
    E --> H
    F --> H

Preventing the Spread of Disease

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

Prevention Method	How It Works	Diseases It Helps Prevent
Hand washing	Removes pathogens from skin before they can be transferred	Almost all communicable diseases
Vaccination	Stimulates the immune system to produce antibodies before infection occurs	Measles, influenza, COVID-19
Isolation / quarantine	Separates infected individuals to prevent spread	TB, measles, COVID-19
Using tissues / covering mouth	Reduces airborne droplets when coughing or sneezing	Influenza, common cold
Safe food preparation	Cooking food thoroughly and storing it at correct temperatures	Salmonella, E. coli
Clean water supply	Treating water to kill pathogens before consumption	Cholera, dysentery
Insect nets / insecticides	Prevents contact with vectors such as mosquitoes	Malaria
Sterilising equipment	Kills pathogens on surgical instruments and laboratory equipment	Hospital-acquired infections

The Body's Non-Specific Defences

Before pathogens even reach the inside of the body, several physical and chemical barriers try to stop them:

Defence	Type	How It Works
Skin	Physical	Acts as a continuous barrier; when skin is cut, blood clots quickly to seal the wound
Nose hairs and mucus	Physical	Trap particles and pathogens in the airways before they reach the lungs
Trachea and bronchi (cilia and mucus)	Physical	Cilia are tiny hair-like structures that sweep mucus (containing trapped pathogens) up towards the throat to be swallowed
Stomach acid	Chemical	Hydrochloric acid (pH ~2) in the stomach kills most pathogens that are swallowed
Tears and saliva	Chemical	Contain the enzyme lysozyme which breaks down bacterial cell walls

Exam Tip: These defences are called non-specific because they work against all pathogens, not just one particular type. The immune system (covered in a later lesson) provides specific defence targeted at individual pathogens.

How Pathogens Make You Feel Ill

Pathogens do not cause disease deliberately — they are simply reproducing. The symptoms you experience are caused by:

Cell damage — viruses destroy cells as they burst out; bacteria can damage tissues as they multiply.
Toxin production — many bacteria release toxins (poisonous chemicals) that damage cells and tissues.
The body's immune response — inflammation, fever, and swelling are caused by your own body fighting the infection, not by the pathogen itself.

It is important to understand that feeling ill is often a combination of pathogen damage and the body's own defence mechanisms at work.

Conditions for Bacterial Growth

Bacteria reproduce most rapidly when conditions are favourable. Understanding these conditions helps explain why certain hygiene practices are important:

Factor	Optimum Condition for Growth
Temperature	Most human pathogens grow best at 37 degrees Celsius (body temperature)
Nutrients	Bacteria need a source of glucose and other nutrients to fuel binary fission
Moisture	Bacteria need water for chemical reactions; this is why drying food can preserve it
pH	Most bacteria prefer a neutral pH (~7); stomach acid (pH 2) kills most bacteria
Oxygen	Many bacteria require oxygen (aerobic), though some can survive without it (anaerobic)

Under ideal conditions, a single bacterium can divide by binary fission every 20 minutes. After just 6 hours, one bacterium could theoretically produce over 250,000 bacteria. This explains why infections can develop so quickly once pathogens enter the body.

Summary

Communicable diseases are caused by pathogens and can spread from one organism to another
The four types of pathogen are bacteria, viruses, fungi and protists
Bacteria produce toxins; viruses invade and destroy host cells
Pathogens spread via direct contact, airborne droplets, water, vectors, body fluids, and contaminated food
The body has non-specific defences including skin, mucus, cilia, stomach acid, and lysozyme in tears
Prevention methods include hygiene, vaccination, isolation, 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 pick up all available marks.