Every day, some half a million computers are infected by bots, according to data compiled by PandaLabs in its annual report on malware activity in 2007. Bots are programs that operate as agents for a user or another program to simulate human activity, and malicious bots can take complete control of the infected system. Once they have control over several hundred computers, cyber-criminals can connect them to create botnets.

Approximately 11 percent of computers around the world are part of these botnets, and they are responsible for 85 percent of all spam sent. In 2007, PandaLabs uncovered several tools such as Zunker or Barracuda, which were being used by cyber-criminals to administer networks of thousands of infected computers across more than 50 countries.

"The creator of the botnet, or the 'bot herder,' can rent out the network to the highest bidder. Cyber-criminals that hire these networks for a wide range of criminal activities including downloading malware onto infected computers, causing denial of services, or the most frequent activity: sending spam," said Luis Corrons, technical director of PandaLabs.

According to the report, spam was one of the major threats during 2007. In fact, more than 50 percent of email received by home users was spam, while the percentage in corporate environments ran at between 80 and 95 percent.

"It would take around 2,000 terabytes of disk space to store the total volume of spam sent in one day," said Corrons.

The subject matter and origin of this junk mail is varied. Sexual health was the most popular theme during the first quarter of 2007, when it accounted for 54 percent of all spam detected. In the second and third quarters, pharmaceuticals topped the list, with percentages of 45 percent and 30 percent respectively. 60 percent of spam is sent from Russia, while 23 percent comes from the USA. Turkey (6 percent), Germany (4.7 percent) and the UK (3 percent) are other countries from which a significant amount of spam is sent.