I know it's an apples and oranges comparison, there have been at least a dozen times in the last twenty years I've exerted myself too hard in the gym (mostly cardio) after a long layoff and got sick right away. Stomach bug or fever. The most recent was in January when I took a few weeks off after getting a really dirty flu strain (apparently this winter's version was hard to recover from) and after 30 minutes of treadmill, I went back to square one the next day and couldn't move. I was out of commission essentially the rest of the month.