I wouldn't now, but I have in the past. I was SUPER sick, but I had taken anti-biotics so I was no longer contagious... But I still felt horrible. I had a trip coming up that next week (where a bikini was needed), so I was there first thing in the morning for cardio, AND I'd go back in the evening for a second workout for that whole week prior to. But no, usually I would skip the workout if I was sick.