The prefix (when used) identifies the type of virus or malware it is. W32 or Win32, for example, denote that it is a Windows 32-bit infector and thus impacts Windows 95, 98, 2000, 2003, XP, Me, NT 4.0. Those that impact only Windows 95/98 often have prefixes of W95. Other vendors apply prefixes that are more indicative of the type of threat, rather than the platform it infects. For example, a TROJ prefix implies the file is a Trojan Horse, an I-Worm prefix indicates it is an Internet/email worm, and OM signifies that it is a Microsoft Office macro virus.
W97M, WM, X2KM are other examples of macro virus prefixes that denote both the fact that it is a macro virus and provides clues as to what versions of Office (or products within Office) are impacted. For example, an X2KM prefix in a virus name indicates that it is a macro virus impacting the Office 2000 version of Excel.
The prefix is usually separated from the name by an underscore, a period, or a slash.
Following the prefix is the actual name of the malware. For example, W32/Bagle has a prefix of W32 and the worm itself is dubbed Bagle.
Many viruses belong to the same family but are slightly different. To differentiate between these variants, antivirus vendors assign an alphabetical suffix. The original virus (or worm, Trojan, etc.) generally does not have a suffix assigned until after further variants of the same threat are discovered. For example, W32/Bagle became W32/Bagle.A after the 'B' variant was discovered.
Subsequent variants are assigned descending letters of the alphabet, i.e. Bagle.A, Bagle.B, Bagle.C through to Bagle.Z. When the end of the alphabet has been reached, the count starts over. Thus, following Bagle.Z will be Bagle.AA, Bagle.AB, Bagle.AC, etc. The third pass through the alphabet would begin with Bagle.BA, Bagle.BB, Bagle.BC, etc. This will repeat as many times as necessary. As of October 2004, the prolific Gaobot variants had reached W32/Gaobot.BOW.
The suffix is generally separated from the virus name by either a period or a dash.
Some vendors also add a modifier after the suffix that further describes what type of malware it is. For example, @mm signifies a mass-mailing email worm and @dl is used by some to designate a downloader.
Using the above information, we can quickly see that W32/Bagle.BB@mm is a Bagle variant that is a mass-mailing email worm impacting Windows 32-bit systems.
One virus, many names
It's one thing to understand how the name is constructed, but what if you are looking for information on the threat? It's important to remember that different vendors assign different names to the same virus. Thus when searching for information on a particular virus, it is imperative that both the vendor and the virus name be referenced.
For example, if using a search engine to find information on Bagle.AT, make sure you also include the name of the vendor that identified it as such. Otherwise, a generic search on Bagle.AT could lead you to information that did not pertain to the particular virus your antivirus software had identified. What Trend Micro calls WORM_BAGLE.AT is W32/Bagle-AU to Sophos, W32/Bagle.bb@mm to McAfee, Win32.Bagle.AQ to Computer Associates, and I-Worm.Bagle.at to Kaspersky. Antivirus vendor Symantec not only considers it a different variant, they also have assigned a different name to the worm family. Instead of Bagle, Symantec persists in calling the family Beagle, thus the Bagle.AT variant used in this example is W32.Beagle.AW@mm to Symantec.