Hello,
Agreed with above comments.
In SEPM 12.1, the "Still Infected" number will go down automatically as the threat is completely removed from the network.
This is a part of the enhanced management console. The management server resets the Still Infected Status for a client computer once the computer is no longer infected. It gives a more accurate status for how many client computers really are infected.
Check this Article:
Cannot Delete the "Still Infected" Value From the Symantec Endpoint Protection Manager 12.1 Console
http://www.symantec.com/docs/TECH165846
Secondly, I would suggest you to work on these Articles:
Identifying the infected and at-risk computers
http://www.symantec.com/docs/HOWTO80990
Remediating risks on the computers in your network
http://www.symantec.com/docs/HOWTO80936
Hope that helps!!