The Storm worm has been around for a while and it has infected a lot of computers since the outbreak on the 15th of August. It changes tactics from time to time to fool people and keep spreading. The main way of Storm worm distribution is spam emails that are usually infected or lead to infected websites.
According to security experts on MessageLabs, the amount of Storm worm email messages has been extremely large and it even raised spam level by 30 percent on the day of outbreak. Luckily, the scale of Storm worm spam decreased later; and it influenced monthly spam amount by 3 percent only.
The Storm worm caused spam spike because it is constructed to send infected email messages from every computer it hits. This way every infected machine becomes a part of botnet that serves to spread the worm further.