What the fuck is this?

This article from the Washington Post is weird and scary.

NORMANDY, Mo., Oct. 23 -- Students at a suburban St. Louis high school headed to the gymnasium for HIV testing this week after an infected person told health officials that as many as 50 teenagers might have been exposed to the virus that causes AIDS.

How in the hell do fifty teenagers at the same school get exposed to HIV? The only possibilities that come to my mind are too appalling to contemplate for long.