According to MacRumors, Apple has updated its malware definitions to address a PDF trojan that gained widespread attention last week. While reports indicated that the trojan's damage was limited to installing a backdoor in users' systems, Apple has moved relatively quickly to address the threat anyway.
CNET reports that yet another OS X trojan is making the rounds, however, this time posing as an Adobe Flash installer. Avoiding this bit of malware is simple: if you must install Flash on your system, only download it from a trusted source like Adobe's own site or MacUpdate. A similar bit of malware made the rounds in August, but Apple updated malware definitions to address the threat; it's likely the company will do the same to squash this newest trojan.
Your Mac's malware definitions are supposed to auto-update, but if you're not afraid of diving into the command line you can force your Mac to update manually.