What is Viruses

Why do people make these viruses and worms? What do they gain by sending worms and viruses?
Originally viruses were mostly the product of individuals who just wanted to cause trouble, make a name for themselves by breaking things, or who were trying to get some kind of revenge against someone else. They did it for "fun", for recognition among their peers, or "just because".

While those folks are still out there I don't believe that they represent the majority of virus sources any longer. We're now seeing the "professional" virus writer emerge.
Here's how that works:

• Someone creates a virus that does two things: propagates itself (more on that in a second) and installs well-hidden remote-control software on the machines it infects. A "zombie" machine or "bot machine" has just been created that can be controlled by the virus writer.
  
• Once enough zombies are created the virus-writer has an army of machines at his command - often referred to a botnet or zombie army. The virus writer is now a "bot herder" as he is able to control or "herd" that network of machines.
  
• He then sells the "services" of his network of zombies. Typically to spammers.
  
• The zombie network then starts sending out massive amounts of spam.
  
• The spammers make money because just enough people actually buy the stuff that spam is selling.
  
• The anti-virus companies eventually identify and block the virus that started all this.
  
• The virus writer writes a new virus, and the process starts all over again.