2. Prevalence - How many people are affected?

3. Points to consider

• Note that prevalence is a proportion and not a rate, although the latter term is often used. So, the term "prevalence rate" is technically incorrect because there is no time period included.

• Prevalence is a useful measure of the situation of a disease. Knowing about the prevalence of a specific disease can help us to understand the demands on health services to manage this disease in a population.

• Prevalence changes when people with the condition are cured or die. Bear in mind that increased prevalence doesn’t necessarily mean a bigger problem. Higher prevalence could mean a prolonged survival without cure or an increase of new cases, or both. A lower prevalence could mean that more people are dying rather than being cured, a rapid recovery, and/or a low number of new cases.

• Prevalence is often used when measuring chronic diseases which are characterised by a long duration and difficulty in determining the exact date of onset.