In the 1980s, authors of the first computer virus, Brain, designed it to destroy only illegal copies of their software. Unfortunately, Brain took on a life of its own and actually started to reproduce itself. Things moved more slowly then, though -- you had to install a new diskette into the computer to get it to reproduce [source: Goodwins].
It didn't take long for more malicious hackers (often called crackers) to see the potential in viruses. Using the Internet to spread, a well-crafted virus can infect millions of computers amazingly quickly and do a lot of damage. The I Love You virus mentioned earlier infected more than 50 million computers and billions of dollars were spent fixing the problems it caused. And that was just one virus.
A virus does this kind of damage basically by inserting itself into the actual code of a program. Whenever the infected program is run, the virus reproduces and tries to infect other computers. To do this, it needs permission from the operating system as well as plenty of other computers running the same software. Under the right circumstances, a virus can infect any computer.
So if viruses are so clever and can infect anything, why haven't they infected more Macs?