Just had dinner with friends. They said a classmate just got diagnosed with chronic fatigue syndrome. I didn't know what to say. Why would someone get that?

Just commiserate. This is a mysterious condition in which seemingly normal persons develop severe fatigue. It is often overdiagnosed, and sometimes may actually be a manifestation of depression or other illnesses. There is some suggestion that human herpesvirus 6 may cause this, but unfortunately there is no known effective therapy at present. There also very few ways to prove the diagnosis.