One last hidden achievement question.

[Willa Grey]

I've got one last achievement locked, and after looking it up, it appears to be The more the merrier" .
I'm guessing this requires multiplayer?

[Kyle]

Simutaneously experience 3 diseases and 1 booster effect (Hard mode only)
That's the text in my global achievements tab.

You can check hidden achievements by going to 'View my achievements' on the game in your library & pressing global achievements, they can be clicked to unhide.

I haven't done it, but I'd probably try Bees (Meningitis), Mantis (Some weird blood disease) & one of those centipede-esque critters that just hang from gardens/etc and attack you, inflicting pyro. (Hypothermia booster, too.)

[ivan.zahariev]

I did it in single player mode, but you need to be on hard difficulty.