3. Incidence

1. Incidence

Prevalence looks at existing cases, both new and pre-existing, while incidence looks at new cases that develop within a given time period.

With short lived illness, like the common cold, incidence and prevalence tend to go hand in hand. It is also possible for a disease to have a high prevalence but low incidence if for example you were to take the case of an infection which is incurable (or long lasting) but does not readily kill, and then develop a highly effective vaccine. The prevalence would still be high due to pre-existing cases, but incidence would be low as the vaccine is preventing new cases from occurring.

For example, HIV is nowadays a treatable infection with a normal life expectancy. This means that with stable numbers of new cases, prevalence numbers will increase. This is illustrated in the following graph.

Figure 1: Comparison of the incidence of HIV (the number of new cases reported each year, shown in blue) with the prevalence (the total number of cases observed each year, shown in red)[1].