Page "Prevalence" Paragraph 5

To illustrate, a long term disease that was spread widely in a community in 2002 will have a high prevalence at a given point of 2003 ( assuming it has a long duration ) but it might have a low incidence rate during 2003 ( i. e. lots of existing cases, but not many new ones in that year ).

Conversely, a disease that is easily transmitted but has a short duration might spread widely during 2002 but is likely to have a low prevalence at any given point in 2003 ( due to its short duration ) but a high incidence during 2002 ( as many people develop the disease ).

As such, prevalence is a useful parameter when talking about long lasting diseases, such as HIV, but incidence is more useful when talking about diseases of short duration, such as chickenpox.