Always get a flu (cough, cold, sore throat, yellow phlegm) within a week after exercise. a 1 hour gym class and i'm sick. how to prevent this pls?

Exercise/infection: There is some evidence that parameters of immune function can be slightly diminished with intense exercise but this is more than balanced by the beneficial effects of exercise. The probability here is that you are being exposed to classmates with infections that are transmitted to you, and not that your immune system is being compromised.