The sick chits (known as profiles for those in the Army) are a source of contention every year. The main problem is parents see them on their children and don't know what's going on, so they always think the worst. As mentioned above most are for legitimate reasons (sprained ankles, bad blisters, etc) and some are for reasons that aren't so clear.
The way a sick chit is issued is a candidate must go to Patten Infirmary for one reason or another. Sometimes somthing happens during PT and they are taken there by the EMS squad, other times they go there during sick call in the morning. They will then be examined by one of the staff members and a determination as to status is made. They then consider what may cause more damage or prevent recovery and prevent that candidate from doing those things.
If the injury is bad enough the parents will hear about it one way or the other, so if you haven't heard anything from the school it's probably no big deal, usually some type of sports injury. With the recent changes (past several years) to indoc the rate of injury has actually decreased and the types of injuries have become less severe.
In the past there were always several admissions to the hospital every indoc; however, these are now few and far between from what I am told and saw at the Indoc.