Mosquito on human skin

Going viral

How diseases spread

Disease spreads when the body’s defensive barriers are breached. Bacteria or viruses might be transmitted directly through contact with infected bodily fluids such as blood, saliva or semen. Some viruses, like influenza, survive just long enough in saliva and nasal secretions that they can be transferred by a sneeze, whereas HIV can’t, and is usually spread through sex or the sharing of needles used to inject drugs. 


The death toll of malaria globally in 2012

Infectious agents can also be transmitted indirectly by other organisms – so-called vectors like the female Anopheles mosquito that carries the malaria parasite. The death toll of malaria (over 620,000 people globally in 2012) has led to the mosquito being described as the world’s most dangerous animal.

Not all diseases cause symptoms straight away, meaning that there may be a window for potentially spreading the infection without realising. This ‘incubation period’, the time between infection and symptoms starting to show, varies between infections: it is a few days for flu, but a few weeks, months or even longer for HIV.

Lead image:

John Tann/Flickr CC BY


About this resource

This resource was first published in ‘Immune System’ in January 2015.

Health, infection and disease, Immunology
Immune System
Education levels:
14–16, 16–19, Continuing professional development