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A pathogen is a microorganism that causes infectious disease: viruses, bacteria, protists or fungi. Free notes with worked examples and 14 questions answered.
Types of Pathogens and How They Spread is part of Communicable diseases in GCSE AQA Higher Biology.
4 min
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17
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GCSE AQA Higher
Pathway
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A pathogen is a microorganism that causes infectious disease: viruses, bacteria, protists or fungi. Free notes with worked examples and 14 questions answered.
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Use this lesson to stay inside Communicable diseases while connecting the detail back to the wider Infection and response topic.
Read the core explanations from Types of Pathogens and How They Spread before testing yourself from memory.
Your body is under constant attack. Right now, millions of microscopic invaders are trying to get inside you, multiply, and make you ill. These disease-causing microorganisms have a name: pathogens.
Think of pathogens as microscopic pirates. They board your body (the ship), steal your resources, and cause chaos. But not all microorganisms are pathogens — many bacteria in your gut actually help you digest food.
Trap: Many students think all bacteria are harmful. Actually, most bacteria are harmless or even beneficial. Only pathogenic bacteria cause disease.
Pathogens come in four main types, each with different characteristics and ways of causing disease. Picture them as different types of invaders, each with their own battle strategy.
Viruses are like hijackers. They can't reproduce on their own, so they invade your cells and force them to make more viruses. Examples include influenza, COVID-19, and the common cold.
Pathogenic bacteria are like invading armies that set up camp in your body. They multiply rapidly and release poisonous toxins. Examples include Salmonella (food poisoning) and Streptococcus (sore throat).
Pathogenic protists are like parasites that live off their host. The malaria parasite Plasmodium is carried by mosquitoes and destroys red blood cells.
Pathogenic fungi are like mould that grows where it shouldn't. Athlete's foot fungus grows on skin, while rose black spot affects plant leaves.
Understanding how pathogens spread is crucial for preventing disease. There are three main transmission routes, and many pathogens use multiple methods to maximise their chances of finding new hosts.
Direct contact is like passing a baton in a relay race. The pathogen moves directly from one person to another. HIV spreads through sexual contact and blood, while athlete's foot spreads through touching contaminated surfaces.
Water acts like a highway for pathogens. Cholera bacteria contaminate water supplies, causing severe diarrhoea when people drink infected water. Plant pathogens can also spread through irrigation water.
Airborne transmission is like invisible clouds of infection. When someone with flu coughs, they release thousands of virus particles that others can breathe in. Fungal spores can travel for miles on air currents.
Plants and animals face different challenges when it comes to pathogen transmission. Animals can move away from infected areas, but plants are stuck in one place and must rely on other defence strategies.
Animal transmission examples:Influenza virus: Spreads through air droplets when infected people cough or sneezeSalmonella bacteria: Spreads through contaminated food and waterAthlete's foot fungus: Spreads through direct contact with contaminated surfacesMalaria protist: Spreads through mosquito bites (vector transmission)
Plant transmission examples:Tobacco mosaic virus: Spreads through direct contact between plants or contaminated toolsCrown gall bacteria: Enters through wounds in plant tissueRose black spot fungus: Spores spread through air and water splashPotato blight protist: Spores carried by wind and rain
Key difference: Plants cannot move away from pathogens, so they rely more on chemical defences and physical barriers like waxy cuticles and bark.
Once you understand how pathogens spread, you can work out how to stop them. Each transmission method has specific prevention strategies that target the pathogen's route of infection.
Preventing direct contact transmission:Regular hand washing with soapUsing barrier methods (gloves, condoms)Isolating infected individualsSterilising medical equipment
Preventing water transmission:Water treatment and chlorinationProper sewage treatmentBoiling water in areas with poor sanitationUsing clean irrigation systems for crops
Preventing air transmission:Wearing masks in crowded areasGood ventilation in buildingsCovering mouth when coughing/sneezingQuarantine during outbreaks
Worked example: Worked Example: Applying Types of Pathogens and How They Spread
Use this method to answer an exam-style question about what are pathogens?.
Answer: A complete response links mechanism, evidence, and conclusion for types of pathogens and how they spread.
Pathogen questions often test your ability to link cause and effect. Examiners want to see that you understand not just what happens, but why it happens and how to prevent it.
Command word focus:'Describe': State what happens in sequence - 'The virus enters cells, reproduces, then bursts out''Explain': Give reasons with 'because' - 'Hand washing prevents transmission because it removes pathogens from skin''Suggest': Apply your knowledge to new situations - think about transmission routes
Common exam mistakes to avoid:Confusing pathogens with diseases (malaria is the disease, Plasmodium is the pathogen)Saying viruses are living organisms (they're not - they can't reproduce independently)Forgetting that plants can get diseases tooMissing that some pathogens use multiple transmission routes
Published by Revise Right. Use the official specification reference to check the requirements for your course. Content updated: 2026-03-20.
Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
A pathogen is a microorganism that causes infectious disease. Pathogens may be viruses, bacteria, protists or fungi and can infect plants or animals, but not all microorganisms are pathogens: most bacteria are harmless or even beneficial.
A protist is a single-celled organism with a nucleus. Some are parasitic pathogens that live inside or on host organisms, such as the malaria parasite Plasmodium, which is carried by mosquitoes and destroys red blood cells.
There are four types of pathogen: viruses, bacteria, protists and fungi. Viruses reproduce only inside host cells, bacteria are single-celled organisms that can produce toxins, protists are single-celled organisms with a nucleus such as the malaria parasite, and fungi can be single-celled yeasts or multicellular moulds.
Most bacteria are harmless or even beneficial, and only pathogenic bacteria cause disease. Many bacteria in the gut actually help digest food, so thinking all bacteria are harmful is a mistake.
Pathogens spread by three main routes: direct contact, water and air. HIV spreads by sexual contact and blood, cholera bacteria spread in contaminated water, and flu spreads through air droplets when infected people cough or sneeze, while fungal spores can travel for miles on air currents.
Viruses are not living organisms, because they cannot reproduce independently. A virus is a non-living pathogen that can only reproduce inside host cells, containing genetic material (DNA or RNA) surrounded by a protein coat; it invades cells and forces them to make more viruses.
Pathogens spread through the air in droplets produced by coughing, sneezing or breathing, or as spores carried by wind. When someone with flu coughs, they release thousands of virus particles that others can breathe in, and fungal spores can travel for miles on air currents.
A pathogen is the microorganism that causes a disease, while the disease is what makes the host ill. For example, malaria is the disease and Plasmodium is the pathogen; confusing the two is a common mistake.
The four types of pathogens are viruses, bacteria, protists and fungi. Viruses reproduce only inside host cells, bacteria are single-celled organisms that may produce toxins, protists are single-celled organisms with a nucleus such as the malaria parasite Plasmodium, and fungi can be single-celled yeasts or multicellular moulds.
Plant pathogens spread by direct contact, wounds, air and water. Tobacco mosaic virus spreads by contact between plants or contaminated tools, crown gall bacteria enter through wounds, rose black spot spores spread through air and water splash, and potato blight spores are carried by wind and rain.
Pathogens spread through water when water sources are contaminated, affecting organisms that drink the water or come into contact with it. Cholera bacteria contaminate water supplies, causing severe diarrhoea, and plant pathogens can spread through irrigation water.
Pathogens spreading through the air can be prevented by wearing masks in crowded areas, good ventilation in buildings, covering the mouth when coughing or sneezing, and quarantine during outbreaks.
Plants cannot move away from pathogens, so they rely more on chemical defences and physical barriers like waxy cuticles and bark. Plant pathogens include tobacco mosaic virus, crown gall bacteria, rose black spot fungus and potato blight.
Examples of diseases caused by each type of pathogen are influenza and COVID-19 (viruses), Salmonella food poisoning (bacteria), malaria (protist) and athlete's foot (fungus). In plants, examples include tobacco mosaic virus and rose black spot fungus.