10.1.3—Transmission of cholera, malaria, TB, HIV

Transmission is successful only when a pathogen leaves an infected source, remains able to infect within a suitable route or medium, and reaches a susceptible new host. The route must connect the source to the host; contact alone is not enough.

• Source and exit: the pathogen must be present in an infected person, animal or body fluid and reach an exit route such as faeces, droplets or blood.
• Viable route or medium: the pathogen must survive long enough in the relevant medium or carrier. Cholera depends on contaminated water or food; malaria uses a female Anopheles mosquito as a vector; TB commonly uses inhaled droplets; HIV requires direct exchange of infected body fluids.
• New host exposure: the route must deliver the pathogen to a person who can become infected. The same labels are not interchangeable: a mosquito is a carrier of Plasmodium, a droplet is an inhaled medium for TB, and casual contact does not provide the body-fluid exchange required for HIV.

Transmission therefore depends on the combination of pathogen, route, environmental or contact conditions and host exposure. Conditions such as contaminated water, crowded air spaces or access to infected blood can increase opportunities for the matching route, but they do not change the pathogen into a different type.

A disease is not transmitted simply because its pathogen is present: the pathogen must reach a new host by a compatible route. This card explains the conditions that make transmission possible; the four disease-specific route chains are established in the neighbouring card, while prevention, diagnosis and treatment are separate learning tasks.