With the apparent disappearance of the Srizbi botnet following the shutdown of McColo late last year, spam levels dropped dramatically. But merely two weeks later spam began rising once again and, according to MessageLabs, have reached levels close to those experienced prior to the infamous hosting firm’s takedown. Apparently, the Srizbi botnet was for the most part killed off after the McColo shutdown but a number of successors are already spreading rapidly and sending out increasing amounts of spam and malware.

In its monthly Intelligence Report, the Symantec owned security firm claims that spam levels are up 4.9 percentage points since December 2008 to account for nearly 75 percent of all email sent on a daily basis. The Mega-D (also known as Ozdoc) botnet is making the largest single contribution to junk mail levels, sending more than 26 million spam emails per minute, while the Cutwail (Pandex) remains the largest active botnet with more than 1 million active IPs this month.

Another new worm that has been infecting PCs at a fast rate is Downadup (also known as Conficker and Confick) but so far it has apparently restricted itself to setting up botnets and is yet to send out any spam or carry out an attack.