From NewScientist: May will go down in history because of the Sasser and Netsky worms,” says Graham Cluley, an analyst at Sophos. Sasser accounted for 51 per cent of all viruses detected by Sophos, with Netsky making up 28 per cent. Sasser spread more quickly than any previous virus mainly because it did not need to be emailed to pass from one machine to the another.