If I have a fever and GI symptoms (vomiting, diarrhea) then I stay home - I've had that for several days on a backcountry trip and it's miserable. But if symptoms are mild and limited to the upper respiratory tract I wouldn't hesitate to go. Aerobic exercise, especially running or vigorous hiking, seems to help keep my lungs, nose and sinus clear when I have a cold, and being outdoors makes me feel better. If you feel better when you're outdoors, you'll probably feel better when you're hiking slightly sick. Some years ago, when she was undergoing chemo for cancer, I took my SO backpacking, hiking, climbing, and scuba diving regularly throughout her treatment. Aside from having to wear extra layers because she lost weight and hair during the chemo, she enjoyed it and it helped keep her spirits up - but she's always been an outdoor girl.