Earlier this week, the Bagle malware authors used an approach of creating a number of new viruses and spamming them massively in the wild.  Each new wave of infected emails contained a different variation of the virus which was designed to elude detection by AV vendors.  F-Secure set an all-time record will 11 releases in one day 

Excellent Writeup by F-Secure on September 20th

 Email-Worm.Win32.Bagle.cy (aka Bagle.BI)
 Email-Worm.Win32.Bagle.cz
 Email-Worm.Win32.Bagle.da
 Email-Worm.Win32.Bagle.db
 Email-Worm.Win32.Bagle.dc
 Email-Worm.Win32.Bagle.dd
 Email-Worm.Win32.Bagle.de
 Email-Worm.Win32.Bagle.df