We’ve all been there. You are in a great routine of consistent exercise and making progress and you get sick. As a trainer I always have people ask the question of whether or not to exercise. One thing for me is how bad do you feel. There are different levels of colds that sometimes you should sit it out and other times go for it.
I find if I have had a cold for a day or two, it does me wonders to get back to a lighter workout just to get the blood flowing and get moving again. There are, of course, limits especially when the illness is more severe and we simply would be irresponsible to our health to get that workout in.
So much comes down to common sense and knowing what we can and should do so we enhance our health and not take away from it.
http://www.webmd.com/cold-and-flu/features/exercising-when-sick