@bythewell - Actually, I would guess most lip infections are due to people not taking proper care of cracked lips, and so they would know exactly where the infection came from.
I had a friend who I went traveling with in a very dry country and his lips did not do well there. I think it was because he broke his nose when he was younger and can't really breath through it comfortably, so he was constantly breathing through his mouth and it dried his lips right out in that desert air.
It was very difficult to keep his lips healthy. I think the only thing that kept them from getting really infected is that he was already on antibiotics as an antimalarial medication.
Some people are prone to cracked lips even when they aren't traveling, and they probably get lip infections all the time. It would just be very difficult to prevent them, I think.