Infection Steam Achievement

I'm not sure if there is a thread for this somewhere that I'm not seeing, and if so I'm sorry.

One of the 3 achievements I still need is Infection. Is there anyone who can help me out with this?

This thread can also be a gathering place if someone else needs this achievement (or a place to direct people to the proper thread if it does exist).

If you check out the Steam forums, there's usually a thread there with people actively responding.

Thanks! Will do.