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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).
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.
AQA requires you to know four types of pathogen:
| Pathogen Type | Size | Living? | How It Causes Disease | Examples |
|---|---|---|---|---|
| Bacteria | ~1–5 μm | 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 μm | Yes — eukaryotic | Often parasites; frequently spread by vectors | Malaria (Plasmodium) |
Pathogens cause disease in two main ways:
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.
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
Bacteria reproduce by binary fission — splitting into two identical daughter cells. Under ideal conditions this can happen every 20 minutes.
N=N0×2n
Where:
For example, starting with 1 bacterium dividing every 20 minutes for 6 hours (18 divisions):
N=1×218=262,144 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) |
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) |
| 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 |
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.
A food-safety technician samples a cream pastry left on a warm counter. At 08:00 the sample contains 50 bacteria per gram. The bacteria double every 20 minutes. Calculate the number of bacteria per gram at 11:00 and comment on the food-safety implication.
Working:
Evaluation: The count has risen from 50 to 25,600 in three hours. Even a very small initial contamination becomes dangerous at room temperature. This is why cream products must be stored in a refrigerator at or below 5 °C — cold temperatures slow enzyme-controlled reactions and extend the lag phase of the growth curve.
A school nurse must advise on two outbreaks: one of measles in a classroom and one of salmonella in the canteen. Explain which control measures would differ and why.
For measles, the pathogen is a virus transmitted by airborne droplets, so isolation of symptomatic pupils, ventilation of classrooms and checking MMR status are priorities. For salmonella, the pathogen is a bacterium transmitted by contaminated food, so hygiene audits, cooking-temperature checks (at least 75 °C core temperature for poultry) and separate utensils for raw and cooked food are essential. The two outbreaks share hand-washing as a control measure but differ in the primary route of transmission.
| Criterion | Bacteria | Viruses | Fungi | Protists |
|---|---|---|---|---|
| Cell type | Prokaryotic | Not a true cell | Eukaryotic | Eukaryotic |
| Typical size | 1–5 micrometres | 20–300 nanometres | Highly variable | 10–100 micrometres |
| Reproduction | Binary fission | Hijack host cell | Spores and hyphae | Often via a vector |
| How damage is done | Toxins and tissue damage | Host cell lysis | Enzyme release into tissue | Parasitic destruction |
| Named Trilogy example | Salmonella | Measles, HIV, TMV | Rose black spot | Plasmodium (malaria) |
| Treatable with antibiotics? | Yes (if not resistant) | No | No | No |
Students regularly confuse three similar-sounding terms. Commit the following precise definitions to memory.
Examiners expect you to describe viruses as non-cellular or acellular. Avoid saying they are "alive" or "dead" — they exist in a biological grey zone because they cannot reproduce without hijacking a host cell. The correct technical phrase in a six-mark answer is: "Viruses are non-cellular infective particles that replicate only inside host cells."
graph TD
P[Pathogen reaches body surface] --> R{Route of entry?}
R -->|Skin cut or wound| S[Bloodstream]
R -->|Inhaled droplet| L[Lungs and airways]
R -->|Swallowed in food or water| G[Gut]
R -->|Sexual contact| M[Mucous membranes]
R -->|Vector bite| B[Bloodstream via bite]
S --> D[Systemic infection]
L --> D
G --> D
M --> D
B --> D
Use this diagram to plan exam answers on how a specific disease enters the body. Every named Trilogy disease slots into one of these entry points.
Exam-style 6-mark question: Describe the four main routes by which pathogens are transmitted between organisms. Evaluate how the route of transmission influences the choice of control strategy. (6 marks — AO1/AO2/AO3)
Grade 3-4 model answer (60-90 words):
Pathogens are germs that cause disease. They can spread when people cough or sneeze on each other. They can spread in dirty food or water. Some are passed on through sex. Mosquitoes can also bite you and pass things on. To stop them spreading you can wash your hands, cook food properly, use condoms or wear nets. Different germs spread in different ways, so people use different methods to stop them.
Examiner commentary: Routes are listed (AO1) and basic prevention given. No precise terminology and no real evaluation, so the answer stays in the lower band.
Grade 5-6 model answer (60-90 words):
Pathogens spread by four main routes: **airborne droplets** (e.g. measles via coughs and sneezes); **direct contact** (e.g. athlete's foot, gonorrhoea); **contaminated food or water** (e.g. salmonella); and via a **vector** (e.g. malaria via *Anopheles* mosquito). Each route suggests a control: vaccination for airborne viral disease, hygiene and barrier contraception for contact, food preparation standards for ingested pathogens, and vector reduction for parasitic diseases. Matching strategy to route increases effectiveness.
Examiner commentary: Clear AO1 with four routes and AO2 link to controls. Limited AO3 evaluation, so middle band.
Grade 7-9 model answer (60-90 words):
*Plasmodium* is transmitted by the female *Anopheles* mosquito acting as a **vector**. The protist invades red blood cells, so a robust **pathogen-specific antibody** response is hard to mount and **herd immunity** via vaccination is not yet reliable; draining stagnant water to reduce breeding remains the most effective population-level intervention. Bacterial pathogens reproduce by **binary fission** so antibiotic stewardship matters. Not every exposure causes infection, because non-specific defences (skin, mucus, stomach acid) block most pathogens.
Examiner commentary: Precise AO1 terminology, sustained AO2 across pathogen types and AO3 evaluation of why route shapes strategy. Top band.
Communicable disease is not only a medical topic. It shapes history, economics and food security. The Black Death of the fourteenth century, caused by the bacterium Yersinia pestis spread by fleas on rats, killed a third of Europe's population and accelerated social change. Agricultural epidemics such as potato blight caused mass migration in the nineteenth century. Modern examples include COVID-19, which demonstrated how a newly emerged virus can disrupt global systems and highlight inequities in healthcare. Appreciating this breadth helps you evaluate exam questions that link biology to social, ethical and economic considerations. A strong candidate explains that control of communicable disease is a shared responsibility between individuals (hygiene, finishing antibiotic courses), institutions (hospital infection control, poultry vaccination) and governments (clean water, vaccination programmes, international cooperation via the World Health Organization).
Use this drill before every exam. Cover the right-hand column and try to generate each answer from the cue on the left.
| Cue | Expected answer |
|---|---|
| Four pathogen groups | Bacteria, viruses, fungi, protists |
| How bacteria cause disease | Produce toxins and damage tissues |
| How viruses cause disease | Replicate inside host cells and cause lysis |
| Formula for bacterial growth | N=N0×2n |
| Main route for Salmonella | Contaminated food |
| Main route for measles | Airborne droplet |
| Main route for HIV | Body fluids |
| Vector for malaria | Female Anopheles mosquito |
AQA alignment: This content is aligned with AQA GCSE Combined Science: Trilogy (8464) specification section 4.3 Infection and response — specifically 4.3.1.1 Communicable diseases, 4.3.1.2 Viral diseases, 4.3.1.3 Bacterial diseases, 4.3.1.4 Fungal diseases and 4.3.1.5 Protist diseases. Assessed on Biology Paper 1.