In the past three months, twelve million new computers have joined botnets, according to data from the latest quarterly McAfee Threats Report.

The number of new bots, identified by IP addresses, represents an increase of nearly 50 percent from the last quarter of 2008, according to the report.

Some good news, however, comes from the report: Spam as a percentage of total email has decreased below the 90 percent mark, which is the lowest level since 2006. And compared with the same quarter a year ago, spam volumes are 20 percent lower this year.

Typically, March is a record month for spam; this year it was well off its normally torrid pace. In 2008, there were an average of 153 billion messages daily, but during this year, March averaged only about 100 billion messages a day.

Spam volume had fallen after the McColo shutdown last year, but the number of new zombies indicates that spammers are working hard to recoup their losses. That means that spam volumes could grow well beyond prior levels soon.

Although spam command-and-control systems are suffused worldwide, China and the United States are home to the largest number of zombie machines under spammer control, with the United States in the lead — spammers seem to continue favoring U.S. computers to send spam email from. Of all countries measured, the top 10 nations, which include the United States, China, Australia, United Kindom, Russia and other industrialized countries, contribute nearly 70 percent of total spam in the world.