To be sick and still come to work. This is usually done to avoid having to unnessicarily use up a sick day
Doing this tends to share your illness with the entire office.
From the biohazard term hot meaning an infectious agent or area, ie a hot agent or a hot zone.
Wife: You have the flu, honey. The proposal can wait.
Husband: Nah, I'm gonna work hot. Don't want to waste my sick days.