Trojans and viruses are generally something Mac users don’t have to worry about, but there’s a lot of hubub about the so-called Flashback trojan that has apparently infected a several hundred thousand Macs worldwide. The trojan takes advantage of a vulnerability in an older version of Java that allows it to download malware which then “modifies targeted webpages displayed in the web browser.”
http://osxdaily.com/2012/04/05/how-to-check-for-the-flashback-trojan-in-mac-os-x/