⚠ Viruses
Viruses live and reproduce inside cells, which causes cell damage.
Communicable Diseases » Pathogens and How Disease Spreads
What you'll learn this session
Study time: 30 minutes
AQA spec: 4.3.1.1
- What pathogens are and the four types: viruses, bacteria, protists and fungi
- How pathogens spread in animals and in plants
- How bacteria and viruses make you ill
- How the spread of disease can be reduced or prevented
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Unlock This CourseWhat is a pathogen?
You already know that a communicable disease is one that can pass from one organism to another. The things that cause these diseases are called pathogens.
Key terms:
- Pathogen: a microorganism that causes infectious disease.
- Toxin: a poison made by a pathogen that damages tissues.
- Vector: an organism, such as an insect, that carries a pathogen from one host to another.
Pathogens may be viruses, bacteria, protists or fungi. They may infect animals, including humans, and they may infect plants. Pathogens are microorganisms, so they are too small to see as single organisms without a microscope.
The four types of pathogen
🔬 Bacteria
Bacteria may reproduce rapidly inside the body and some make toxins.
💧 Protists
Some protists are pathogens. They may infect animals or plants.
🍄 Fungi
Some fungi are pathogens. They may infect animals or plants.
Named examples of diseases from each group are taught in the lessons Viral Diseases and Bacterial, Fungal and Protist Diseases.
How pathogens spread
A pathogen has to get from an infected organism into a new one. Pathogens can be spread in three main ways.
- Direct contact: touching an infected person, animal or plant, or touching a surface they have contaminated. In plants, a healthy leaf can touch an infected leaf.
- Water: pathogens can be in drinking water or in water used for washing food. Rain and water in the soil can also carry pathogens to plants.
- Air: tiny droplets from a cough or sneeze carry pathogens. Some plant pathogens make tiny spores that are blown from plant to plant on the wind.
Pathogens can also be spread by vectors. An insect that feeds on one infected organism and then on another can carry the pathogen between them.
👨 Spread in animals
Droplets from a sneeze, dirty hands, unclean water and contaminated food all move pathogens between people and animals.
🌿 Spread in plants
Wind blows spores, rain splashes pathogens from leaf to leaf, soil water carries them to roots, and insects carry them between plants.
How pathogens make you ill
Once inside the body, a pathogen can make you ill in different ways.
Reproducing rapidly
Bacteria and viruses may reproduce rapidly inside the body. A very small number of pathogens that get in can quickly turn into millions. The more there are, the more harm they can do.
Bacteria and toxins
Bacteria may produce poisons called toxins. Toxins damage tissues and make us feel ill.
Viruses and cell damage
Viruses are different. A virus gets inside a cell and uses the cell to make more viruses. So viruses live and reproduce inside cells, causing cell damage. A damaged cell cannot do its job properly.
Worked example
A patient has a sore throat. Tests show that bacteria are growing in the throat tissue. Explain why the patient feels ill.
Answer: the bacteria reproduce rapidly, so there are many of them. They produce toxins that damage the tissues of the throat, and this makes the patient feel ill.
Reducing and preventing the spread of disease
You may be asked to explain how the spread of a disease can be reduced or prevented. Think about breaking the route from one organism to the next.
🧼 Hygiene
Washing hands, washing and cooking food properly, and keeping surfaces clean remove pathogens before they can be passed on. Covering your mouth when you cough stops droplets spreading through the air.
🚫 Isolation
Keeping an infected person or animal away from others stops direct contact. For plants, infected plants can be separated from healthy ones.
🐛 Destroying vectors
If an insect carries a pathogen, killing the insect or removing the places where it breeds cuts off the route of infection.
Vaccination can also prevent the spread of disease. A vaccinated person is protected, so they are far less likely to pass the pathogen on. You will study how vaccines work in the lesson on Vaccination.
Clean water supplies also stop pathogens in water from reaching people.
Worked example
A farmer finds one plant with a disease that spreads when leaves touch. The farmer moves the plant away from the healthy plants. This stops the pathogen reaching them by direct contact.
Common mistakes
Saying that all pathogens are bacteria. There are four types. Saying that every pathogen makes toxins: it is bacteria that may make toxins, while viruses cause damage by living and reproducing inside cells. Writing that hygiene "kills the disease" instead of saying it stops pathogens being passed on.
Exam-style question
Some pathogens are spread by drinking dirty water.
(a) Name the four types of pathogen. [2 marks]
(b) Explain how bacteria in the body can make a person feel ill. [3 marks]
(c) Suggest two ways to reduce the spread of a disease that is passed on by drinking dirty water. [2 marks]
Model answer
(a) Viruses, bacteria, protists and fungi (1 mark for two correct, 2 marks for all four).
(b) Bacteria reproduce rapidly inside the body (1). They may produce toxins/poisons (1). The toxins damage tissues and make the person feel ill (1).
(c) Any two from: provide clean or treated drinking water (1); keep drinking water supplies away from waste (1); wash hands before eating or handling food (1); wash food in clean water (1).
Exam tip
In part (b), the toxins must be linked to damaged tissues. Do not just write that bacteria "make you ill".