scout camp, whatever, get wet in rain storms, don't sleep, eat junk food, knock yourself out horsing around, hiking etc... ...not a problem. Then you get home tired and weary and boom, catch a cold. Never understood that. Maybe your body is dumping adreniline into the system while you're there, fight or flight mode or something, keeping you going. Get home and you get into shut down/repair mode. You are also exposing yourself to a lot more people again so maybe it's that.