Clamscan whole disk or certain places?
As I'm the only user who has access to root (and hence am unlikely to load a virus onto the server because my workstation's virus scanner would detect it first), the only other sources of new files (excluding package updates) would be via vhosted domain ftp usage. Since each virtual host is chrooted to their folder structure, only the /var/www folder structure would need to be scanned. And since the rest of the server is essentially static, periodic scans would suffice.
SpamAssassin handles viruses arriving via email.