Alopecia.
Alopecia is hair loss or balding. It may happen on any part of the body. There are many types of alopecia. Some types cause temporary hair loss and your hair will grow back. With other types, hair loss can get worse, and become permanent.
Knew a dude whose whole body went bald when he was in his 20's. Went to bed and woke up in a pile of his own hair.