The average number of attacks by Conficker on computers has grown from 15 in the first quarter of 2011 to 35 in the fourth quarter. The worm emerged in late 2008 and made a major splash in the early part of 2009 when researchers discovered that the worm had an update scheduled for April 1. The update was just a new algorithm that would provide new C&C servers for the infected machines. Microsoft s research found that the huge majority of Conficker infections 92 percent comes from stolen or weak passwords.
Source: https://threatpost.com/microsoft-conficker-attacks-rebound-042512/76483/