Top 20 viruses for July

Despite Klez topping the virus charts again this month, experts have said that the month of July saw virus infection reports on the decrease for the first time this year.

"In July, we finally saw a slight decrease in the number of tracked viruses from a prior month," said Steven Sundermeier, product manager at Central Command.

But it is thought that the reduced infections may be a product of the summer season, and that the trend may not continue. "This may be simply because more users are on vacation and away from their computers," Sundermeier said.

More recent virus discoveries were hot on the heels of Klez through July in the shape of politically motivated virus Yaha and the Frethem worm.

"Yaha came to prominence ... launching a rudimentary denial of service attack on the Pakistan government's website and encouraging Indian hackers and virus writers to join forces and attack Pakistan," said Graham Cluley, senior consultant at Sophos.

Frethem relies on a simple bit of social engineering to get the victim to open an attachment that claims to contain "your password".

Also noteworthy for July is the growing list of worms exploiting security holes in the Kazaa network and other file-sharing and peer-to-peer networks.