depends how i feel. feverish - usually stay home. not too feverish / weak and nose isn't running too badly...
but be a lot gentle with myself with respect to my performance.
feels to me like i sweat things out in a good way (like eating lots of chili when i have a cold). increase in symptoms, sure, but i think those symptoms are what the body uses as a defense against sickness (trying to flush the system out) so you recover faster.
sleep great afterwards.
i also drink a lot of water when i'm at the gym but probably tend to run a bit dehydrated otherwise. so the fluids probably help me feel better. but also increase the flushing things out symptoms.
i do feel a bit guilty when i ring in sick for work and hit the gym, though...
just feeling generally run down (as opposed to fighting off a cold / flu specifically) is sometimes an indicator that it is deload time. i try and listen to that. hard to know what is best, sometimes.
maybe i'm wrong... but i'm starting to think that when i really don't feel like training... then i shouldn't train. that doesn't happen very often. so when it does happen it is time to listen.