Left 4 Dead Outbreak Achievement?

There’s been a lot of chatter about the Outbreak Achievement in Valve’s latest success Left 4 Dead (Thanks again Valve, you’re super.) The achievement is very vague in its conditions, and has spawned some debate over its meaning.

The Outbreak achievement’s description reads as follows:
Catch a rare strain of infection, then pass it on to someone else.

This is very ambiguous in its interpretation, while every other achievement appears to have a very straightforward requirement. It’s been speculated that the achievement occurs when a survivor makes contact with another player while covered in Boomer bile. Others say all developers start with the rare infection and “tag” it to other players as they play throughout the community.

I think we all respect Valve here. We recognize them as smart and playful developers. People who like playing and making games. So for them to just fuddle around with a single achievement’s definition is simply out of character for them. I’d guess that a handful of players started with the infection in their Left 4 Dead games, and are passing the “infection” to other players on their steam lists. I think it’d be very interesting to see Valve post up some stats on how quickly we were all infected. I think it’d also be very cool to see who didn’t get the achievement. In other words, who didn’t become infected, and are ultimately, the infection survivors.

If you’re reading this, what are your thoughts on this whole Outbreak thing? What do you suspect it could be?