I suspect that, like Giardia, infection depends on exposure to more than some minimum number of agents, and the state of preparedness of the immune system. Imagine taking a liter of water from a polluted stream. One hiker drinks a third of a liter and is OK, the other drinks the rest and we'll leave the results to your imagination. Like Hantavirus, Giardia has a lengthy incubation time.
So we have a situation where, for some reason, the hantamouse population doubles, so any exposure is twice as intense, and the chance of infection is increased exponentially.
This is an interesting discussion, but there is an 800-pound gorilla in the room which hasn't even been mentioned...
Disclosure: I didn't stay at an Express-something hotel last night.
