As long as you don't have a fever working out when you are a bit under the weather can be beneficial as exercise increases metabolism and infection fighting agents.  Physical activity increases inner body core temperature and if you are already feverish - you'll be adding insult to injury.  Speaking of injuries, if you cannot use your normal range of motion (arms and legs) you should not conduct that particular activity - seek out low-non-impact related alternatives like swimming, cycling and aqua jogging/exercises. 

So many strategies! - I'm sick and tired of making excuses! - How 'bout you?