Macs have generally been unpopular, and the reason they don't get viruses is because hackers almost never
write viruses for Macs. Think about it. If you write a virus, you want it to become prevalent, widespread. What are you going to target? Until only a few years ago(before Linux went mainstream and Apple stepped up their advertising and got cutthroat), Windows had the overwhelming majority of market share. Even now, as I recall, the numbers still favor Windows, if not as a majority then at least a plurality. Windows PCs are cheaper than Macs, and as an operating system it's better known and easier to get than Linux. Naturally, they'll get more attention. Macs were never particularly more secure than Windows, they just never got any
action attention.
Where I get my info specifically, however, is
Pwn to Own, a contest which sets up three PCs: one Linux, one Mac, one Windows, and challenges hackers to comprimise the computers' security to win prizes.
Last year, one of the teams shocked tech gurus by announcing that the Mac system had been easier to hack than the one running Windows Vista. When the dust had settled that year, the Mac was comprimised on day 2, the Vista machine day 3, and the Linux machine had been ignored because it just wasn't a challenge. Think about it: Linux is an open-source operating system. Creating a backdoor would be as easy as one of the contributors sneaking one into a release candidate build. The team that did it said that he'd been expecting Windows to fold like a house of cards, and had focused the entirety of Day 1 on it. But it didn't. The Mac had proven easier to hack.
This year, the same team made headlines by gaining control of a Mac within, as I recall, less than a minute from green light to total control using a vulnerability in Safari(Safari on Mac is the least secure browser/OS combination available, currently. The best is Chrome on Vista.). Now yes, there was a good deal of research and preparation involved, but once he had that done, he had a way to go from discovering a new Mac to adding it to a botnet in less time than a user would need to realize that something's up. In the aftermath of this year's competition, he was interviewed, and asked what the best choice for security would be. I effectively paraphrased his answer in my previous post.