Over 600,000 Macs are part of a botnet.

The Russian company Dr. Web released an article yesterday stating that over 550,000 (the number has risen since then) computers running OS X are infected with a malware which steals personal information. It seems like 274 of the infected computers are located in Apple's headquarters.
The malware was first discovered in September 2011 when it disguised itself as a Flash Player plug-in installer. It has changed a bit since then and is now taking advantage of a Java vulnerability on OS X. The strange thing is that Oracle have had a patch for this since February, but Apple just recently started sending it out (can be found here). As far as I know, the malware installs itself (without you getting prompted for a password) as soon as you visits an infected site. F-Secure has instructions on how to check if you're infected, and also how to remove it, which can be found here: Trojan-Downloader:OSX/Flashback.I.