The first malicious computer program that has spread outside the computer system it was written in was Elk Cloner. It was written around 1982 by a 15-year-old high school student for Apple II systems.

The first PC virus, called Brain, was discovered in the wild four years later, in 1986. The origins of the Brain virus are disputed. Allegedly, it was created by a Pakistani software firm to deter pirated copies of software they had written.

The term “computer virus” was first used in 1984 by academic publication Fred Cohenin his paper Experiments with Computer Viruses.

Some other notable viruses were CIH, also known as Chernobyl or Spacefiller, in 1998, and VBS/Loveletter worm, also known as the "I love you" virus in 2000.

The Chernobyl virus was one of the most harmful widely circulated viruses, destroying all information on users' systems and in some cases overwriting the system BIOS.

The I Love You virus that caused widespread e-mail outages was the most costly virus to business, causing upwards of 10 billion dollars in damage.

There are more than 150,000 computer viruses in the world today. And many countries have adopted new laws to deal with the authors of malicious computer programs. For instance, in the UK, if you're caught using a computer for criminal reasons you can receive a five-year sentence and an unlimited fine.

A guide to computer viruses