"Every day we are detecting more than 250,000 connecting to the internet and sending mail," said Paul Judge, chief technology officer at Ciphertrust.
"That's unique machines that have never done it before," he said. "It's a distribution platform that is becoming more popular for attackers."
Machines that have become part of botnets (fleets of bots) can be very hard to spot, and as numbers have increased, those behind the botnets have started to specialise in the uses to which the botnets are put. The hiring out of such botnets remains a pretty risky business, though, with tough punishments for those involved.