Intersting post, considering all the sinus shit I've been dealing with for years. It is very hard to stay away from the gym for me, but still I will not go when sick with certain things. IMHO:
Don't go when:
Running a fever
Flu-like symptoms, causing body aches, nausea, vomiting
Feeling absoulutely shitty with something else, ie severe cold or sinus infection
Do go when:
A minor common cold
Nasal congestion, etc.
Two days after fever subsides
Do shorter less intense workouts. Exercise stimulates the immune system, but our usual intense bodybuilding workouts can ovrtax it, so just do a shorter stimulating workout. I find that when my allergies or sinus issues are blocking up my nasal passges, once I get training, they open right up--physioligically, they expand, as do the bronchial tubes/lungs. Plus all those hormones the body pumps out act as anti-inflammatories. Cool shit!