I have a friend of mine which whenever she goes abroad she always comes back ill every single time. I find it quite funny
I, too, always get sick either towards the end of the trip or as soon as I come back home. I think it has something to do with doing too much, not getting enough rest, and not eating well for an extended period of time. Even though I KNOW I'd end up sick, it doesn't stop me from travelling. And even though I have the above theory of why I get sick every time, I somehow cannot stop myself from my excesses. I blame it on adrenalin. Adrenalin does give you more energy to do more, sleep less, and I'm pretty sure that it cuts your appetite and weakens your immune system.