Forget about "sweating out" a cold or flu – don't train if you have flu! Strain on an infected heart muscle (myocarditis, one of the possible complications of flu) may lead to paralysis of this muscle. There have been cases – in young sportspeople – where the only way to survive after myocarditis was a heart transplant. A gradual return to exercise after recovering from flu is best. You should generally not exercise for at least seven days after recovering.
Read more: Should you get a flu shot?