To me it's when I can't function properly. Quite honestly so many people get sick throughout the winter, and you can't always say it's because of so-and-so. The fact is there's always someone in the office sneezing or coughing at least... And to say that 1 sick person equates to epidemic in the office, that's going too far. Are we all this weak?
Sometimes it's a minor cold I wake up with and I feel better the next day already. If I'm to take off for even a minor symptom because I'm afraid of getting the office sick, I'd be out of sick leave/PTO already.
The fact is most colds you can take some OTC medicine and you'll be fine. My worst cold last winter was dealt with using Mucinex. Sure I coughed up some nasty crap and blew my nose full of crap sometimes, but I was decent enough to work and Iw asn't coughing every 30 seconds. If it's a flu or something I'd take a day or two off at most, but you can't expect to stay out until you completely recover. Just make sure the worst is past. Some medication and you should be fine in the office.
Stomach pains are a different issue. I walked straight out 2 weeks ago when I was just in a whole load of pain and compounded by the fact I spent the whole weekend doing photography and slept in some weird position the night before, my back was in pain. I couldn't even sit up in my desk, so sometime in the afternoon I took off early and collapsed on my bed for 5 hours (barely made it home with all the stomach pain).