Why do computer viruses exist?

Why do computer viruses exist?

Computer viruses were originally created by programmers for fun, to annoy other programmers on the network in the 1980s and to confront them with tricky tasks. Back then, the commercial Internet did not exist like it does today. As a result, these viruses might cause little or no damage. With the growing popularity of the Internet among private users beginning in the late 1990s, hackers increasingly released viruses. Since then, the goals of computer viruses have been very different. While some use computer viruses to protest against companies or institutions, others use viruses to deceive Internet users.

All the differences between viruses, worms and Trojans are simply explained here.