It's a trojan horse, not a virus. Those aren't unknown on OS X, just rare--I remember maybe ten or so over the years. The last Mac virus was SevenDust, which was for the original Mac OS. When I was doing QA for Norton AntiVirus for Mac in 1999 I had to test with it, and I still remember it very well because it was such a nuisance to work with. The darn thing was buggy, and wouldn't infect reliably.
