ClamAV (Clam anti-virus) is a suite of programs designed to detect malware, viruses, trojans, and other threats. It be run from the command line or as a daemon, and can integrate with mail servers to scan email attachments. It is open source and published under the GPL.

Homepage: http://www.clamav.net/
Bug reports: https://bugs.launchpad.net/ubuntu/+filebug

From the apt-cache description:

ClamAV is an open source (GPL) antivirus toolkit, which is designed to detect trojans, viruses, malware and other malicious threats.
 The package provides a flexible and scalable
 multi-threaded daemon in the clamav-daemon package, a command-line
 scanner in the clamav package, and a tool for automatic updating via
 the Internet in the clamav-freshclam package. The programs are based
 on libclamav6, which can be used by other software.
 This package contains the daemon and its command line interface,
  - fast, multi-threaded daemon;
  - easy integration with MTA's;
  - support for on-access scanning;
  - remote scanning;
  - able to be run supervised by daemon.
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