Clam AntiVirus is a GPL anti-virus toolkit for UNIX. The main purpose of this software is the integration with mail servers (attachment scanning). The package provides a flexible and scalable multi-threaded daemon, a command line scanner, and a tool for automatic updating via Internet. The programs are based on a shared library distributed with the Clam AntiVirus package, which you can use with your own software. Most importantly, the virus database is kept up to date.
There are two commercial antiviral software solutions for OS X, but they both have terrible reputations for performance issues, system damage, etc. They cause more problems than they prevent. Fortunately OS X viruses are still very rare, but this is not a tenable situation.
Some OS X experts are implementing ClamAV, a UNIX antivirus solution. Apple should incorporate this into Tiger and support the project.