It has been widely believed by many that Macs are more secure than Windows PCs and are less likely to become infected by a virus. It is more accurately believed by fewer people that it is purely a matter of numbers, and virus writers target Windows PCs because there are so many more of them.
There have been more and more instances of a virus being found on a Mac, and now according to Ivan Sorokin, a malware analyst at Dr. Web a botnet named ‘Flashback’ is infecting over 600’000 Macs, of which 247 are reportedly in Cupertino – presumably a reference to Apple headquarters.
The Flashback trojan first surfaced around October 2011 and the most recent version, which can become installed if you visit a compromised or malicious webpage, exploits a Java vulnerability which has been subsequently patched by Oracle.