A Russian security company, Dr. Web, have said they've stumbled upon a botnet with over 600,000 infected Macs.

More than 50% of the infected computers are found in the US, along with 19% in Canada and 12% in the UK. 

The virus is known as "BackDoor.Flashback.39" and if you think your computer might be infected, I highly recommend you check out their website. To protect your computer, install Mac's latest security update.