Ever wonder where spam comes from? In the old days, spammers would rent or buy their own Net access directly; the problem was, angry antispam geeks would track them down and complain to the ISPs, who would kick the spammers offline.

The next technique was to create “zombie” computers — to send out worms and viruses that infect everyday computers and instal spam-sending relays. This way, a spammer can create thousands or millions of zombies which he or she can use to send spam. I heard a lot about this back when I was writing my feature on virus-writers for the New York Times Magazine; police told me the spammers were potentially linked to organized crime in China and Russia. But everyone wondered precisely how many “zombie” victim computers there were.

Finally someone’s measuring it. Comcast, the Internet cable-access giant, has started tracking how many messages are sent out by their users. Sean Lutner, a network engineer, told CNET that Comcast users send out 800 million messages a day — but only 100 million go through the company’s official servers. The other 700 million other ones are thus probably spam sent out by Comcast users infected with spam relays. They probably don’t even know they’re doing it — yet according to those stats, the average American family is sending out six to seven pieces of spam a day.

How did they generate these statistics?

“It’s not rocket science,” John Levine, co-chair of the Internet Engineering Task Force’s antispam research group, said of this technique. “Basically, you count the mail, and you give everyone a quota. If Grandma usually sends six messages a day and now tries to send 10,000 messages a day, what are the odds that she made that many new friends?”

(Thanks to Slashdot for this one!)