Here's an article from Macworld describing malware that can infect a Mac. Jim Hamm writes, "To be safe, I disabled Java in Safari preferences. If one should need Java occasionally for, say, a bank site, just enable it for that site."
Today, 4-5, David Passell sends along this link to the BBC claiming that more than half a million Apple computers have been infected with Flashback Trojan.
If you do a search on Apple Discussions watch for the date to see if it's current information. Here's one link about it.
Now, an article on how it works, how to detect it, and how to remove it.
Today, 4-6, Jim Hamm writes that he's used the trojan-check from Mac2School's Ruth Davis. She spoke to PMUG awhile back. You can email her here for the very important virus information.