How often do you get sick? I usually catch some kind of virus maybe once or twice a year (thankfully), and I use this sick time to take a break from the gym and supplementation. We all know that mentally it is hard to stop lifting, but when you feel like crap you arent getting any gains. If you dont get sick often use the time as a break and muscle memory will be your best friend when you get back... and your cholesterol levels will prob be back to normal too. I recently took 2 months off to work more (which sucked) and I was back up to my ideal weight and strength in 2 weeks.
The stomach bug is very contagious and it seems to run through houses rampant. Each year it gets a little bit worse too, I have seen people actually pass out from it. I dont think any prevention can help with avoiding it.
As far as other viruses are concerned I've noticed through out the years that heavy supplementation (creatine, NO2, protein, and an MVI) actually makes my immune system crap out occasionally. Maybe this is one of the precursers to you getting sick.