From the Health Dashboard

If you have the required account permissions, you can monitor health problems and athlete availability instantly by selecting the Health option of the Team menu. A table automatically displays all athletes with health problems. 

A red row means complete unavailability, and a yellow row means that the athlete can participate but with limitations.  The Comment section will display the content of the last medical note.

The athlete availability status can be changed by opening the health problem (click on it's name in the table) and selecting the appropriate option in the Training / competition status. 

From Team Charts

To generate a graph that displays the number of percent of available/unavailable athletes over time, open Team Charts, select a period, and scroll down to the Availability section