Charles C. Mann reporting on pay-per-click advertising fraud:
Probably the most worrisome emerging threat is zombie networks
— hordes of linked machines controlled by rogue software.
Without their owners’ knowledge, these boxes continuously send
spam, transmit worms and viruses, participate in
denial-of-service attacks, and execute a host of other
antisocial tasks. These zombie networks can be enormous. […]
The robot machines create clicks from all around the world at
apparently random intervals, making them difficult to identify.