Smarmy Mac fanboys always claimed that Macs were immune from viruses because of superior coding. In reality, virus writers chose to ignore the relatively small (4% or so) percentage of Macs and concentrated their efforts on the Windows world.
Now that the Mac is gaining some market share, they are becoming a target.
The Flashback Trojan has compromised an estimated 600,000 plus Macs, making it comparable to the massive Conficker worm botnet that affected Windows PCs.
PC World reports that "According to current data from Net Applications, Mac OS X is the number two desktop OS with 6.54 percent market share. Windows, on the other hand, accounts for 92.48 percent of the market. Based on market share, the Flashback Trojan botnet is equivalent to a Windows botnet of nearly 8.5 million PCs. That makes it an even larger threat than Conficker--just on a much smaller platform."
They continued, "It doesn’t help anything that Apple perpetuates the myth of invulnerability. It takes time to develop a patch, but as soon as Apple was aware that the threat existed, it should have proactively communicated to Mac OS X users to make them aware. In fact, it should have provided users with instructions to disable Java and mitigate the threat pending a patch to resolve the issue. The fact that it didn’t is probably a contributing factor to why the Flashback botnet is as large as it is."