There is a virus for Mac that has recently started to pick up steam (infected over 600,000 Mac computers worldwide). It is called the Flashback trojan, and relies on a vulnerability in your computer's Java installation. This virus operates in the background, so many users don't know they are infected. Click Here for more details, and steps you can take to see if your computer is infected.
Other Mac Viruses
Currently there are multiple malware programs circulating designed to infiltrate Mac OS. These programs are similarly designed to look like legitimate antivirus programs and tell users that their Mac is heavily infected with viruses. The program then harasses people into providing credit card information to purchase fake anti-virus program to remove the infection. This is a ploy to get your personal information.
Often times these programs are automatically downloaded but cannot be installed without the permission of the user, by way of an administrator password. People who are not aware of these malware threats can unknowingly install these programs on to their machines. ONLY install programs on your machine that are from trusted publishers and that you have knowingly downloaded. Also, make sure that you have an updated antivirus program on your Mac. Students and Staff of OSU have access to ClamXav, which is a free antivirus program.
Here is some info from the Apple Support community about removing Mac Defender.