McAfee maintains a database on all the virus they know. If you can find the virus in the http://vil.mcafee.com database, you can find the McAfee virus signature level required to protect yourself (e.g. W32/Nimda.gen requires DAT (virus signature level/definitions) of 4162 or above to protect yourself).
Verify if your mcafee virus signature definitions is update enough (Ref: How to check the version of virus signature and Scan engine?). The last 4 digits is is DAT level. If you have virus definition/signature level is larger than the one required. The virus could be detected by the scanner.