ClamAV definitions out of date
This could be because of a number of reasons. Check that the database is up to date by calling the freshclam command directly with verbose:
fab $environment -H backend-1 sdo:'freshclam -v'
If it reports the virus databases are up to date then you may need to check the ClamAV virusdb archive to investigate.
If it can’t seem to download the updated definition files, with errors
Ignoring mirror 126.96.36.199 (due to previous errors), it
could be that all the mirrors available have been blacklisted. You
can reset the blacklisted mirrors by deleting the file that stores
sudo rm /var/lib/clamav/mirrors.dat. After doing this running
sudo freshclam -v may well work again.