Mac users should be on guard against a fake Mac security tool being distributed online, says the Intego security firm. The program, called MacGuard, claims to scan a computer for "adware, spyware and trojans," and then eliminate them; in reality the app is actually a version of existing Windows malware, which has already infected as many as 30 million people worldwide. The latter assumes control of a person's computer, and displays messages warning about a false infection in an attempt to get users to pay money.
Yep. Even Macs are vulnerable. However, the market is so small that most hackers will not take the time to develop malicious stuff for the platform.
Just be smart when downloading and installing anything. The OS will prompt and warn you when trying to launch something downloaded from the net and will also require you to input your password when installing software.