Sophos, one of the industry leaders in security software, has recently said that malware attacks like viruses, worms and trojans have increased so much for windows systems, that Windows users should consider switching to Mac
They go on to claim that the top 10 forms of malware for the PC are not capable of infecting the Mac OS. I don't think these forms of malware will never surface in a form that's capable to infect Macs, but it's certainly less likely there will be malware for the Mac OS than for Windows.
Sophos' claim has certainly run into a lot of criticism
from users that refuse to believe Macs are any more immune to viruses than Windows, but when windows sets the security bar as low as it does, it's hard for other OSes to not