Machines infected with Conficker.C have effectively stopped previous versions of the worm from spreading. Conficker C shut off the spreading mechanisms in these machines, and installed instructions to check in at 500 rendezvous points, selected randomly from a list of 50,000 web domains, each day, beginning yesterday, April 1. Variant C, which appeared in the wild on March 5, scans the Internet for PCs infected with previous variants, A, B and B++. It s not clear how many machines are infected with each separate variant.
Source: https://threatpost.com/confickerc-stops-earlier-variants-spreading-040209/72532/