By default, the Outbreak Filters feature scans your incoming and outgoing messages for possible viruses during an outbreak. You can enable scanning for non-viral threats in addition to virus outbreaks if you enable anti-spam scanning on the appliance.
https://www.cisco.com/c/en/us/td/docs/security/esa/esa12-0/user_guide/b_ESA_Admin_Guide_12_0/b_ESA_Admin_Guide_chapter_01110.html