clamav

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