Computer got a cold?

Here is another possibly paranoid thought that is related to the last posting. Why would anyone write a computer virus program? There are several reasons that I can think of - but one seems most logical. To sell anti-virus software! I'm certain a good investigation would find a connection between those that write virus programs and those that sell the anti-virus software. There is tons of money to be made on this software - but they need a good scare once in a while to keep it selling. Again, I hope someone can prove me wrong.