In the history of computer viruses, there have been several phases in virus creation:
1) people write viruses simply because the idea of a computer virus is entertaining. These viruses lived on floppy disks and didn’t do much.
2) people write computer viruses becuase they like seeing the virus spead. These were not intended to cause harm, but did cause damage in lost time and by clogging the Internet. An example is the Morris worm, whose source code is on a floppy disk on display in a museum.
3) people write computer viruses becuase they like causing damage. This is the arson and vandalism of the cyber world. These viruses are the ones that wipe hard drives clean. These are also the ones that mess up all of your files and your monitor background.
4) people write computer viruses because they like to steal personal and corporate data. These are designed to steal data and personal information. They do not wipe hard drives, and are the first generation of malware that is designed to not be noticed.
5) people write computer viruses to recruit large numbers of home computers into botnets that are remotely controlled. Once a person has control of a botnet, they can use it for profit, such as by spamming. The computers can also be used to launch denial of service attacks against websites and organizations.
6) countries write computer viruses as an espionage tool against other countries. This occurred with the creation of the Stuxnet, Duqu, and Flame worms. These are designed to record large amounts of important data for military uses. Another potential reason is that computers are often put in charge of machines and robots. If the computer system can be compromised, then one country can take control of the other country’s infrastructure and cause it to break.
One other note is that the word “virus” can be exchanged with “worm”. They are technically separate, but do similar things.