Does SpamAssassin filter email-borne viruses as well as spam?

No. [SpamAssassin] is focussed on identifying spam, and not viruses. The mechanism of scanning for viruses is different, and software tailored for virus scanning is much much better at this task. It is possible that in future versions that SA will include hooks to call out to external virus scanners, but SA itself is unlikely to attempt such functionality itself. You may be interested in \[http://www.clamav.net/ ClamAV\], a GPL virus scanner.

An easy way to use the ClamAV daemon with \[http://www.procmail.org/ procmail\]:

:0wc
| clamdscan -

:0efw
| formail -b -f -t -I "X-Clamav-status: yes"

This will add "X-Clamav-status: yes" to anything message that isn't found to be virus free.