Incidence & Prevalence
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4. Incidence
Prevalence looks at existing cases, while incidence looks at new cases. Reading in a newspaper that a certain country has a prevalence of 2% of a certain disease will not give you any information about when those patients got the disease, or for how long
they have had it.
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. Looking at the new cases (incidence) provides a deeper understanding of what is going
on.
Depending on how incidence is expressed, either as a percentage or per person-year there are two different formulas.


Often
you can read about the incidence of a heath event as a certain number of cases
per 100 or 1,000 person-years.