Every day, about half-a-million computers are infected by bots, programs that reside on computers awaiting commends from their creators – who can then take control of the infected system. Once they've got control of a few hundred computer, cyber-crooks link them up in botnets. 

This is according to data compiled by PandaLabs in its annual report on malware activity in 2007.
Approximately 11% of computers around the world are part of these botnets, and they are responsible for 85% of all spam sent.
In 2007, PandaLabs uncovered several tools such as Zunker or Barracuda, which were being used by cyber-crooks to administer networks of thousands of infected computers across more than 50 countries.
“The bot herder, the creator of the botnet. can rent out the network to the highest bidder. Cyber-crooks that hire these networks use them for a wide range of criminal activities including downloading malware onto infected computers or causing denial of services. Although one of the most frequent activities is sending spam.” explains Jeremy Matthews, the CE of Panda Security (South Africa).
Along these lines, junk mail was one of the major threats during 2007, according to the annual report from PandaLabs. In fact, more than 50% of email received by home users was spam, while the percentage in corporate environments ran at between 80% and 95%.
“To get an idea of the scale of the problem, it would take around 2 000Tb of disk space to store the total volume of spam sent in one day,” says Matthews.
The subject matter of this junk mail is highly varied. Sexual health was the most popular theme during the first quarter of 2007, when it accounted for 54% of all spam detected. In the second and third quarters, pharmaceuticals topped the list, with percentages of 45% and 30% respectively.
In terms of the origin of junk mail, some 59,72% is sent from Russia, while 23,08% comes from the US. Turkey (6,12%), Germany (4,77%) and the UK (3,16%) are other countries from which a significant percentage of spam is sent.