When a large number of hijacked computers are under the control of a single person remotely via a control system and net-based command, this is called a BOTNET.
The number of these computers joining a BOTNET is increasing, as the computers are being hijacked by malicious hackers with one mission (to send out viruses and spam).

There are more than six million computers world-wide that are part of a bot network, this figure is up by around 30% than in 2006 according to a recent survey by security firm Symantec.

One third of all these computer attacks came from computers in the United States and we guess many of the people that own these PC’s have not got a clue they are being used in such a way.
News Source