Travel fever

I don't understand why the girl gets sick, usually with a fever, every single time we go on holiday. Except for that one trip we made when she was ten weeks old, she's come down with something every time we travel. It does not matter if we go by car or plane, stay in a house or a hotel. She always gets sick.

We live in London, so she's not wanting for constant exposure to strange germs. Could travelling have such an impact on her that it weakens her immune system?
I'd potentially blame it on travelling in winter and her dealing with the unexpected chill, but the same thing happened when we went to the beach in August. She eats mostly the same things on holiday. The only notable difference is that we put her in disposable nappies when travelling (rather than reusable cloth). That can't possibly make a difference, can it?

We're going to be in the States for 3 weeks at the end of the month. I'm dreading what sort of cold she'll come up with this time. Must stock up on baby ibuprofen.