PC-welt has a nice article on the latest wave of Worms/Downloader which hit the net in the first week of February: http://www.pcwelt.de/news/sicherheit/71196/index.html.
Here is an excerpt for non German speakers:

With the spam like waves new versions are distributed continuously to prevent detection by virus scanners. This approach is only partly successful, some vendors, like the open source project Clam have developed generic signatures, that are able to detect new variants at a very high level.