Virus War - Netsky v. Bagle v. MyDoom (60 variants in 3 months)
In about 3 months a total of 60 variants have emerged as noted in the link below. The "wars" started when one virus writer started removing infections from the other two families if present. This competition has started a steady stream of new variants almost daily.
HISTORY OF THE VIRUS WARS
http://www.f-secure.com/weblog/archives/vw5.gif