Many people who run Linux or Unix servers think they do not have to concern themselves with anti-virus software because those operating systems are not commonly attacked by viruses. What they fail to realize is that a Linux server, particularly the mail server running on it, can still distribute viruses, effectively acting as a carrier.
Clam AntiVirus or ClamAV is a free anti-virus toolkit that detects and effectively removes viruses before they can infect a system or be passed on to others. It has a command line virus scanner, a database updater that pulls new virus definitions from the central server, and a daemon to run as a background service.
In order to scan incoming mail, ClamAV requires a mail filter, such as MailScanner. ClamAV is free and open source software, released under the GPL. It is available in most Linux distribution repositories, as well as BSD, Solaris, Mac OS X, and other Unix operating systems. There is also a separate organization that maintains a Windows version called ClamWin.