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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.
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).
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 |
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.
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
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 |
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.
Pathogens do not cause disease deliberately — they are simply reproducing. The symptoms you experience are caused by:
It is important to understand that feeling ill is often a combination of pathogen damage and the body's own defence mechanisms at work.
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.
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.