How many Macs are infected with viruses or other malware?
According to British anti-virus software maker Sophos, it's one in five.
The good news, or at least not-so-bad news, is that almost all of the malware found on Macs is actually meant for Windows machines.
It's simply hiding out on Macs, mostly harmlessly, but waiting for an opportunity to spread to a Windows PC — or to the Windows installation found on many Intel-based Macs.
The not-so-good news is that one in 36, or 2.7 percent, of the Macs that Sophos analyzed were indeed infected with Mac-specific malware. Of those, 75 percent were carrying the Flashback family of malware that's spread across the Mac world since mid-March.
The second-most-widespread Mac malware infection was the MacDefender scareware, which was found on nearly 18 percent of Macs. It first appeared in the spring of 2011.
"Some Mac users may be relieved that they are seven times more likely to have Windows viruses, spyware and Trojans on their Macs than Mac OS X-specific malware, but Mac malware is being surprisingly commonly encountered," Sophos' Graham Cluley said in a press release. "Mac users need a loud wake-up call about the growing malware problem."
Sophos analyzed data from 100,000 Macs whose users had installed Sophos' free Mac anti-virus software.
Much of the Windows malware found on the Macs was fairly old, with some samples dating back to 2007. The most prevalent bug, the Bredo Trojan, found on 12 percent of Macs, was first spotted in 2009.
"Cybercriminals view Macs as a soft target, because their owners don't typically run anti-virus software and are thought to have a higher level of disposable income than the typical Windows user," Cluley said. "Mac users must protect their computers now, or risk making the malware problem on Macs as big as the problem on PCs."