Depends on the severity of the illness. I go the the gym with a mild cold, or a bit of a stomach bug and see how it goes. If I feel fine after the warmups then I just carry on with the programme. However, if I'm coughing so hard that I can't get a breath, or I feel terrible, then I go home and nurse myself. You can actually push a virus into the heart if you just tough it out, that would put the training on hold for a long time.