I do regular strenuous training, and this makes sense to me. You are weaker immediately after a strenuous workout than immediately before. That's why adequate recovery time is just as important as workout time. It makes sense to me that immediately after the workout, before your body has had a chance to recover, your immune system is weakened and you're more susceptible to illness.
Personally, what keeps me coming back to my workouts is the challenge. I'll take the extra risk of the occasional cold or flu over the risk of dropping my workouts entirely from disinterest.
Ha, I've never had my workouts compared to smoking. My workouts have many other positive benefits, which I think outweigh the risks. Smoking has no positive benefit other than it's something to do while sitting in bars.
Personally, what keeps me coming back to my workouts is the challenge. I'll take the extra risk of the occasional cold or flu over the risk of dropping my workouts entirely from disinterest.