Today I noticed that I have got the Outbreak Achievement: Catch a rare strain of infection, then pass it on to someone else.
There is some speculation that Valve designated a few players as infected with this rare strain and that these players pass it on to others through some kind of contact, such as covering them in boomer bile, for example. Maybe you have to make "contact" more than once to successfully pass the infection along even?
I remember a couple of days ago getting the Big Drag achievement by snaring the same player three times in a row on a single life as the smoker, so maybe that was when I passed it on? It would make sense to me that the infected are the ones that pass it on - what do you think?