Cambridge O Level Biology · Syllabus 5090 · Disease and Immunity
Indirect Transmission
What is Indirect Transmission?
Transmission in which a pathogen reaches a new host by way of something else: contaminated food, water or surfaces, droplets carried in the air, shared contaminated equipment, or an animal vector that carries the pathogen from one host to another.
This definition is part of the Disease and Immunity chapter in Cambridge O Level Biology.
Questions students ask about Indirect Transmission
Why is malaria classed as indirect transmission when the mosquito puts the parasite straight into the blood?
Because the parasite passes through another organism on the way. Direct transmission means nothing in between; the mosquito is an intermediate, and an intermediate organism is called a vector. The injection at the end does not change the classification, because the parasite still had to travel through the mosquito to get there.
Last reviewed Syllabus 2026-2028