Like the others have shared already.
Practicing good oral hygiene works wonders, wash your hands, don't breath other sick people exhaust, no rubbing the tear ducts, picking the nose or ears, and never bite your nails.
A little extra rest between workout help build up the immune system.
Depending where you are extra carbs are easier to handle than big heavy fats & too much protein also over works the system.
Get healthy and you can train harder with better gains.