Actually, the viral infection Carmichael is describing is a sneaky fellow. It can lead to cardiomyopathy, a condition in which your heart muscle becomes unable to work effectively, usually because it is dilated (though there are other forms of the disease as well). The damage may not show up for years, so that the infection has passed out of memory long before the heart problems show up. My dad died of a heart attack after living with CM for several years. He was a lifelong athlete, but rarely at intense levels, who played or worked out nearly every day - including days when he was not quite over a cold. His cardiologist believed that to be the cause of his disease: since he worked out when his immune system was still under stress from the illness, the CM-causing virus had an easy time taking hold.

Obviously, my personal experience has colored my feelings about working out when ill. I don't. Luckily, working out is the best illness preventive I have found! Since doing regular aerobic exercise, I haven't been sick with anything other than mild allergies.