Six botnets are responsible for 85 per cent of all spam, according to an analysis by net security firm Marshal. The Srizbi botnet is reckoned to be the largest single source of spam - accounting for 39 per cent of junk mail messages – followed by the Rustock botnet, responsible for 21 per cent of the spam clogging up users’ inboxes.

Spam emanating from the Mega-D botnet, which Marshal reckons was the leading source of junk mail in early February, was temporarily stemmed after control servers were taken out in mid-February. The estimated 35,000 zombie clients associated with the Mega-D botnet were infected with the Ozdok Trojan. Most spam comes from just six botnets | The Register