While bulk e-mailers have, in the past, sent unwanted messages from a single server, increasingly the spam emanates from networks of compromised PCs, known as bot nets. The level of junk e-mail has increased almost in lock step with the number of compromised systems used for spam.

What is most alarming is that new clients--Internet addresses that we have never seen before and which could be new infections--have tripled since June," said Hart, who posted a chart tracking the growth on his Web site this week.

Bots and bot nets have rapidly emerged as one of the major threats on the Internet. Tens of thousands of compromised PCs are frequently counted among a single bot net's unwilling members, with some bot nets boasting as many as a million systems. Traditionally, the networks have been used to install adware on victims' machines or level denial-of-service attacks at online companies as part of an extortion scheme.

Now, spammers are frequently counted among the operators or the clients of bot nets. Last May, a spammer only identified as "PharmaMaster" used a bot net to target anti-spam provider Blue Security and its Internet service providers with a massive denial-of-service attack that blocked access to the companies for hours and, in the case of Blue Security, days. Because of the attack, the company exited the anti-spam business.

Many bot herders--as the criminals that infect computers with bot software are named--sell or rent bot nets to others to use, and spammers increasingly seem to be among their customers.

Some Internet users have noticed an indirect effect of the surge in bulk e-mail. Spammers usually put another person's e-mail address in sender's field of the message. Because many spam and antivirus filters send back a rejection message to the sender, the actual owner of the e-mail address will be inundated with replies.

Security researchers that use honey pots--heavily monitored computers that are allowed to be infected by malicious software to spy on the attackers--have also confirmed the connection between bot nets and spam, said Thorsten Holz, a graduate student and the founder of the German Honeynet Project.