From the start until the end of an outbreak investigation,it will remain important that we have an overview of the number of cases that have occurred over time. For this reason we need an exact definition for what we will call a case. We can share this definition with those involved in finding / reporting the cases and those analysing the data.

We may need a different type of case definition for initial outbreak appraisal than for the analytical investigation. For example at the start of the investigation, when we want to find all cases of the outbreak to determine the magnitude of the problem, we may want to use a very sensitive case definition, to avoid that we miss cases. 

Later, during the analysis phase, we usually want to switch to a very specific case definition, in order to avoid misclassification (hence bias our results). These practical aspects of case definitions make that they may evolve during investigation.

Various levels in case definitions are usually the categories possible, probable, confirmed, which may help us to make the case definition more sensitive or more specific.