Communicable diseases: pathogens and transmission

What you'll learn

Communicable diseases are illnesses that can spread from one organism to another. This revision guide covers the four types of pathogens that cause these diseases, how they are transmitted between hosts, and the methods used to prevent their spread. Understanding these concepts is essential for AQA GCSE Biology Paper 1 and forms the foundation for topics on immunity and disease prevention.

Key terms and definitions

Pathogen — a microorganism that causes disease in its host organism

Communicable disease — an infectious disease that can be transmitted from one organism to another through direct or indirect contact

Vector — a living organism that transmits a pathogen from one host to another without being affected by the disease itself

Transmission — the process by which a pathogen passes from an infected individual to an uninfected individual

Bacteria — single-celled prokaryotic organisms that can reproduce rapidly inside the body and produce toxins that damage tissues

Virus — non-living particles much smaller than bacteria that reproduce inside host cells, causing cell damage when they burst out

Protist — single-celled eukaryotic organisms, often parasites that cause disease

Fungi — eukaryotic organisms that can be single-celled or have a body made of hyphae, some species of which cause disease

Core concepts

Types of pathogens

Pathogens fall into four main categories, each with distinct characteristics that determine how they cause disease and how they can be treated.

Bacteria are single-celled prokaryotic microorganisms that lack a true nucleus. Not all bacteria are pathogenic — many are beneficial — but disease-causing bacteria reproduce rapidly inside the body, often producing harmful toxins. These toxins damage tissues and make us feel ill. Examples of bacterial diseases include:

• Salmonella (food poisoning)
• Gonorrhoea (sexually transmitted infection)
• Tuberculosis (TB) — affects the lungs primarily

Bacterial cells are typically 0.5-5 micrometres in size and can be treated with antibiotics, though antibiotic resistance is an increasing concern.

Viruses are not considered living organisms because they cannot reproduce independently. They are much smaller than bacteria (approximately 0.02-0.3 micrometres) and consist of genetic material (DNA or RNA) surrounded by a protein coat. Viruses work by invading host cells and using the cell's machinery to produce many copies of themselves. When these new viruses are released, they damage or destroy the host cell. Examples include:

• Measles (causes fever and skin rash)
• HIV (attacks immune system cells, eventually leading to AIDS)
• Tobacco mosaic virus (TMV) — affects plants, causing discolouration

Antibiotics are ineffective against viral infections. Prevention through vaccination or antiviral drugs are the main control measures.

Protists are eukaryotic organisms, meaning their cells contain a nucleus. Most protists are single-celled. Many pathogenic protists are parasites, living on or inside other organisms and causing them harm. They are often transferred to hosts through vectors. The key example for GCSE is:

• Malaria — caused by Plasmodium species, transmitted by mosquitoes

Protists are typically larger than bacteria and require specific treatments depending on the species.

Fungi can exist as single-celled organisms (like yeasts) or have a body made of thread-like structures called hyphae. Some fungi produce spores that can spread between organisms. Fungal diseases can affect both plants and animals. Examples include:

• Athlete's foot in humans (affects skin between toes)
• Rose black spot in plants (causes purple or black spots on leaves)

Direct transmission routes

Direct transmission occurs when a pathogen passes from one host to another without an intermediary. Understanding these routes is crucial for implementing effective prevention strategies.

Direct contact involves physical touching between an infected and uninfected individual. This includes:

• Skin-to-skin contact (e.g., athlete's foot, some STIs)
• Contact with bodily fluids (e.g., HIV through blood, gonorrhoea through sexual contact)
• Contact between contaminated surfaces and broken skin

Droplet infection occurs when an infected person coughs, sneezes or talks, releasing tiny droplets of moisture containing pathogens into the air. These droplets can be breathed in by people nearby or land on surfaces. Diseases spread this way include:

• Influenza
• Tuberculosis
• Measles
• COVID-19

Droplets typically travel 1-2 metres before falling, which is why social distancing measures are effective in reducing transmission.

Ingestion involves consuming contaminated food or water. Pathogens enter the digestive system and cause infection. Salmonella bacteria, for example, contaminate food (especially poultry and eggs) and cause food poisoning when consumed. Poor food hygiene increases transmission risk.

Indirect transmission routes

Indirect transmission involves an intermediary — either a non-living object (fomite) or a living organism (vector) — that carries the pathogen from one host to another.

Fomites are contaminated objects that transfer pathogens. Common examples include:

• Door handles
• Mobile phones
• Utensils and crockery
• Clothing and towels

Regular cleaning and hand hygiene reduce transmission via fomites.

Vector transmission requires a living organism to carry the pathogen. The vector itself is not usually affected by the disease. The most important example for GCSE is the female Anopheles mosquito, which transmits malaria:

1. The mosquito bites an infected person and takes up Plasmodium protists with the blood
2. The Plasmodium reproduces inside the mosquito
3. When the mosquito bites another person, it transfers the protists into their bloodstream
4. The protists travel to the liver and then red blood cells, reproducing and causing malaria symptoms

Other vectors include houseflies (transmitting various bacteria from faeces to food) and aphids (transmitting plant viruses).

Water and soil can also act as vehicles for pathogen transmission, particularly for bacteria and protist cysts. Contaminated water sources spread cholera and other waterborne diseases, especially in areas with poor sanitation.

Factors affecting disease transmission

Several factors influence how easily communicable diseases spread within populations.

Population density and overcrowding increase transmission rates because:

• People are in closer proximity, facilitating droplet infection and direct contact
• Shared facilities (toilets, kitchens) increase contamination risk
• Ventilation may be poor in crowded spaces

Urban areas typically experience faster disease spread than rural communities.

Climate and weather affect pathogen survival and vector populations:

• Warm, moist conditions favour many pathogens
• Mosquito populations increase in warm climates with standing water
• Cold temperatures may reduce some pathogens' survival outside hosts but increase indoor crowding

This partly explains why malaria is predominantly found in tropical regions and why respiratory infections peak in winter in temperate climates.

Hygiene standards directly impact transmission. Poor hygiene practices include:

• Inadequate handwashing
• Improper food preparation and storage
• Contaminated water supplies
• Lack of sewage treatment

Improved sanitation infrastructure has dramatically reduced communicable disease rates in developed countries.

Prevention and control measures

Controlling the spread of communicable diseases requires interrupting transmission pathways and reducing pathogen loads.

Personal hygiene measures include:

• Regular handwashing with soap — removes pathogens before they enter the body
• Using tissues when sneezing/coughing and disposing of them properly
• Avoiding sharing personal items like towels or razors
• Proper food handling and cooking (particularly for poultry)

Isolation and quarantine separate infected individuals from healthy populations, preventing transmission during the infectious period. This is particularly important for highly contagious diseases.

Vaccination programmes provide immunity without requiring infection, reducing the number of susceptible hosts in a population. When enough people are vaccinated, herd immunity protects even unvaccinated individuals.

Vector control reduces disease transmission by targeting the organisms that carry pathogens:

• Insecticide-treated mosquito nets prevent malaria transmission
• Draining standing water eliminates mosquito breeding sites
• Insecticides reduce vector populations
• Biological control using natural predators

Antimicrobial treatments cure infections, reducing the time an individual can transmit the pathogen:

• Antibiotics for bacterial infections (though antibiotic resistance is increasing)
• Antiviral drugs for some viral infections
• Antifungal treatments

Public health measures implemented by governments and health organisations include:

• Clean water provision and sewage treatment
• Food safety regulations and inspections
• Health education campaigns
• Disease surveillance and rapid response systems
• Travel restrictions during outbreaks

Plant diseases and transmission

Plants also suffer from communicable diseases that can devastate crops and natural ecosystems. Understanding plant disease transmission is important for food security.

Rose black spot is caused by a fungal pathogen that spreads through:

• Water splashing fungal spores from infected leaves to healthy ones
• Wind carrying spores between plants
• Contact between infected and healthy leaves

The disease causes purple or black spots on leaves, which then turn yellow and drop off. This reduces photosynthesis capacity, weakening the plant. Control involves removing infected leaves and destroying them, plus using fungicides.

Tobacco mosaic virus (TMV) affects tobacco plants and related species (tomatoes, peppers). It spreads through:

• Direct contact — farmers touching infected then healthy plants
• Contaminated tools
• Insects that feed on plant sap

TMV causes distinctive mosaic patterns of discolouration on leaves and stunted growth. No cure exists, so prevention focuses on using resistant plant varieties and removing infected plants immediately.

Worked examples

Example 1: Identifying transmission routes (3 marks)

Question: A student wants to reduce their risk of catching influenza during winter. Describe three ways influenza is transmitted and explain one method the student could use to reduce transmission.

Model answer:

Influenza is transmitted through droplet infection when infected people cough or sneeze (1 mark). It can also spread through direct contact with contaminated surfaces (fomites) such as door handles (1 mark). The student could reduce transmission by washing their hands regularly with soap, which removes virus particles before they can enter the body through the mouth, nose or eyes (1 mark).

Examiner note: This answer identifies two transmission routes (2 marks) and explains a prevention method with reasoning (1 mark). Common errors include listing prevention methods without explaining how they interrupt transmission.

Example 2: Comparing pathogens (4 marks)

Question: Compare bacteria and viruses as pathogens.

Model answer:

Bacteria are living cells that can reproduce independently, whereas viruses are non-living particles that require a host cell to reproduce (1 mark). Bacteria are larger (0.5-5 micrometres) compared to viruses (0.02-0.3 micrometres) (1 mark). Bacteria make us ill by producing toxins that damage tissues, while viruses damage cells when they burst out after reproduction (1 mark). Bacterial diseases can be treated with antibiotics, but antibiotics do not work on viral infections (1 mark).

Examiner note: Comparisons require statements about both pathogens. The answer uses comparative language ("whereas," "compared to") effectively. Four distinct differences earn full marks.

Example 3: Vector transmission (6 marks)

Question: Malaria is transmitted by mosquitoes. Describe how the disease spreads from one human to another and explain two methods that could reduce malaria transmission in affected regions.

Model answer:

A female mosquito bites an infected person and takes in Plasmodium protists with the blood (1 mark). The protists reproduce inside the mosquito (1 mark). When the mosquito bites an uninfected person, it injects the protists into their bloodstream (1 mark), where they travel to the liver and red blood cells, causing malaria (1 mark).

Method 1: Using insecticide-treated mosquito nets prevents mosquitoes from biting people while they sleep, interrupting transmission (1 mark). Method 2: Draining standing water removes mosquito breeding sites, reducing the vector population (1 mark).

Examiner note: Four marks available for describing the transmission cycle with sufficient detail. Two marks for prevention methods with explanations of how they work. Higher-mark questions require extended answers with clear scientific reasoning.

Common mistakes and how to avoid them

• Confusing transmission routes with prevention methods. Transmission describes how pathogens spread; prevention describes actions to stop this. Always read the question carefully to determine what is being asked.

• Stating that antibiotics kill viruses. Antibiotics only work on bacteria. Viruses reproduce inside host cells using different mechanisms, so antibiotics are ineffective. Learn which treatments apply to which pathogen types.

• Describing viruses as living organisms. Viruses are non-living particles that cannot reproduce independently. They require a host cell's machinery to replicate, which is why they are not classified as living.

• Forgetting to explain mechanisms in "explain" questions. If asked to explain how a prevention method works, don't just name it — describe the biological process it interrupts. For example: "Handwashing removes pathogens from skin surfaces before they can enter the body through mucous membranes."

• Mixing up vectors and pathogens. The mosquito is the vector (carrier); Plasmodium is the pathogen (disease-causing organism). Vectors transmit pathogens but aren't usually affected by the disease themselves.

• Using vague terms like "germs" instead of specific pathogen types. Use precise terminology: bacteria, viruses, fungi, or protists. This demonstrates scientific understanding and earns more marks in extended answers.

Exam technique for "Communicable diseases: pathogens and transmission"

• Command word recognition is crucial. "Describe" requires factual statements about what happens. "Explain" requires reasoning about why or how something occurs, often using linking words like "because," "therefore," or "this causes." "Compare" needs statements about similarities and differences using comparative language.

• Use mark allocations to guide answer length. Each mark typically requires one distinct scientific point. A 3-mark question needs three separate points or one point developed across three logical steps. Count your points before finishing.

• For disease transmission questions, follow the pathway systematically. Start with the infected host, describe how the pathogen leaves their body, explain the transmission route, then describe how it enters the new host. This logical structure prevents you from missing steps.

• Link prevention methods to transmission routes. If you identify that a disease spreads through droplet infection, suggest prevention methods that target this specific route (masks, ventilation, social distancing). This shows applied understanding and earns explanation marks.

Quick revision summary

Communicable diseases are caused by four pathogen types: bacteria (produce toxins), viruses (reproduce in host cells), protists (often transmitted by vectors) and fungi (produce spores). Transmission occurs directly through contact, droplet infection or ingestion, or indirectly via fomites or vectors like mosquitoes carrying malaria. Prevention methods interrupt these pathways through hygiene, isolation, vaccination, vector control and antimicrobial treatments. Understanding specific examples like TB, HIV, malaria and rose black spot demonstrates how transmission routes determine appropriate control strategies.