Upper Respitory Infection and Exercising

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So I had a pretty serious URI that I am getting over (my husband's turned into pnemonia). When is the best time to get back into exercising? I tried to start exercising a week ago but couldn't catch my breath when I was doing cardio. So now it's been 2 weeks since I last exercised and it's been killin' me! Has anyone else had any experience trying to get back into the exercising routine after an illness? How long does it normally take and how do I know I'm ready?