"The reason for the sudden fall was a coordinated attack on one
of the most successful botnets out there called Rustock. Multiple
servers used by the botnet were taken offline simultaneously on
Wednesday crippling Rustock's 26 command and control networks and
therefore stopping them sending out around 30 billion spam messages
every single day.
"Those networks were thought to be controlling in the region of
815,000 Windows PCs around the world infected with Rustock's
rootkit and doing its dirty work. Unless those command networks can
be brought back online, which nobody wants, those infected machines
become useless to the spammers."