I presume you're referring to Mac viruses. Yes they do exist, but I've never come across them via the net. You can use AV software for auto-detection if you like, & you can setup a firewall if you choose. Personally I only manually check the files (auto-detection annoys me.) See the following discussion which drifts into this topic: http://www.gnutellaforums.com/showth...threadid=26512
I hope this answers your question.