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Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.
Key concept
A pathogen is a disease-causing organism. The four types are bacteria, viruses, fungi and protoctists. Transmissible diseases spread by direct contact, droplet infection, contaminated food or water, and animal vectors. Non-transmissible diseases cannot pass from person to person.
Understanding pathogen types and transmission routes is the foundation of the diseases and immunity topic. Questions typically ask you to name a pathogen type for a specific disease and describe how it spreads: always with the transmission route, not just "it is contagious".
Types of pathogen
Pathogen type
Examples of diseases
Key feature
Bacteria
Cholera, tuberculosis (TB)
Single-celled organisms that reproduce rapidly; some produce toxins
Viruses
Influenza, HIV/AIDS, COVID-19
Non-living particles that invade host cells and use them to reproduce, destroying the cells
Fungi
Athlete's foot, ringworm
May grow as hyphae on or in body surfaces
Protoctists
Malaria (Plasmodium)
Single-celled eukaryotic organisms; malaria protoctist is transmitted by mosquitoes
Transmission routes
Direct contact: skin-to-skin or through body fluids. Athlete's foot spreads through contaminated floors and shared towels. HIV is transmitted through exchange of body fluids.
Droplet infection: pathogens in tiny droplets released by coughing, sneezing or talking. Influenza and TB spread this way.
Contaminated food or water: cholera bacteria spread through water contaminated with faeces from infected people.
Animal vectors: an organism that carries a pathogen between hosts without being affected itself. The female Anopheles mosquito transmits Plasmodium (malaria) when it feeds on blood.
Transmissible versus non-transmissible
Transmissible (communicable) diseases can pass from one person to another: examples include cholera, malaria and influenza. Non-transmissible (non-communicable) diseases cannot spread between people: examples include coronary heart disease, diabetes and cancer.
When a question asks whether a disease is transmissible, focus on whether a pathogen passes between individuals. Genetic conditions and deficiency diseases are not transmissible even though they can occur in multiple family members.
Try a focused question (3 marks)
Cholera is a transmissible disease caused by a bacterium. Describe how cholera is transmitted.
Show the answer and marking guidance
The cholera bacterium is present in faeces of infected people.
Faeces contaminate water supplies, especially where sanitation is poor.
Other people drink the contaminated water and ingest the bacterium, becoming infected.
Original Cambridge-style practice written for this site. It is not an official Cambridge past-paper question.
Common mix-ups to avoid
Check these points against your own answer before moving on.
Saying viruses are living organisms: they are non-living particles that need a host cell to reproduce.
Confusing transmission route with symptoms of the disease.
Calling the mosquito the pathogen: the mosquito is the vector; Plasmodium is the pathogen.
Describing non-transmissible diseases as contagious.