Today there was a lua exploit infecting all source games (gmod, CSS, tf2, l4d, etc.) sort of like a virus.
If you want to be very safe do not play ANY source games.
If you really must, dont join any servers with !!! in the name and dont accept any freind requests on steam untill this is fixed.

A map of people infected: http://freehostedscripts.net/oc.php?id=SUQ0MDY4MDUyfGZyZWVob3N0ZWRzY3JpcHRzLm5ldHwxfA==

Facepunch forum post on the topic: http://facepunch.com/showthread.php?t=1386818

Steam forum post on the topic: http://forums.steampowered.com/forums/showthread.php?p=35468592

BE CAREFULL

This infection can make you spam messages and invites to random people.
The main message is: *cough cough* and there are a few others too

I have been asking arround and heard that this wil most liekly be fixed by tomorrow.