Every customer has a 'Health Status' field in their profile. If there is anything in that field, it will be shown on the attendance taking screen once the customer has been marked as attending the class.
This is particularly helpful for Yoga classes, where it's important to know what poses your clients cannot do. Now the instructor can see any issues in that class at a glance.
The Health Concerns section is on the right side of the attendance screen, below the attendee list.
You can set a health status for any of your customers by going to their customer details page, clicking the Edit link under their name, and entering the status in the Health Status field on the right side of the page.