The infection being detected is on a worm which propagates by some means (such as Autoplay feature of Windows and mapped drives). The worm is probably hiding on computer(s) that has no AV or on a shared drive and repeatedly spreading to the rest of the computers in the network. You need to find this source computer(s) and remove the infection to get rid of it. Please follow the best practice to stop the infection from propagating and take steps to find the source of the infection.
Few best ways to stop/reduce the speed of propagation of the infection:
1) Disable Windows Autoplay feature on all the computers in the network (I would suggest to keep it disabled forever and this would save the network from worms). On computer that are part of domain, this can be done through group policy. On computers that are part of workgroups that has to be done locally.
2) Password Protect Shared Drives (at least until the issue is resolved). DO this especially when you have customers/users who connects through VPN from outside your network.
Identify the source of the Infection and removing the infection:
1) Enable Risk Trace in SEPM (This needs the firewall component of the SEP client to be enabled, hope you have them enabled already). After enabling Risk Tracer, the SEPM would be able to find the sources in a few hours.
2) Ensure that all the computers in the network has the AV software installed and that they have up-to-date virus definition. Comparing the list of computer in the network (domain and/or workgroup) with the list of machines reporting to SEPM to get a list of machine that are not reporting to SEPM. Take necessary steps to make them report to SEPM or ensure that an AV software is installed and up-to-date on them. If you have VPN users/customers, ensure that their computers have AV installed and are up-to-date.
References:
http://www.symantec.com/docs/TECH91705
http://www.symantec.com/docs/TECH122466