A virus is a self-replicating program that moves from one computer to another. The virus makes copies of itself and is propagated via a network or via removable storage (e.g., floppy disks or zip drives). A virus is an executable program that has to be executed in order for it to copy itself.

Early viruses were inevitably used floppy disks to propagate. One of the principal ways in which viruses spread was infected disks used by people who copied programs from one machine to another.

The advent of the Internet has provided a medium across which viruses can spread round the world in hours. In principle, a virus can't spread via email because the host program must be executed and text can't be executed. However, modern applications are very sophisticated and incorporate facilities that allow interaction with the environment such as macros and scripting languages. Of course, these facilities can be exploited by virus writers.

One of the most significant viruses was Love Bug that was transmitted by e-mail on 10 May 2000. This virus has the subject line "I love you" and spread round the world in two hours reaching over 80 million computers and affecting over 2 million. This is the only virus that  seriously infected one of my machines - my normal caution was reduced because the email seemed to come from someone with whom I had been in dispute - I assumed the message was ironic.

The Love Bug virus was harmful because it deleted JPEG images and MP3 files. It was specifically aimed at Microsoft's Outlook Express and used its address book to propagate to other computers.

The effect of Love Bug was so serious that some companies and even government agencies had to shut down their systems.

It is reported that the Love Bug was developed by Onel de Guzman a 23-year-old student in a computer college in Manila, The Philippines. de Guzman denies that he was responsible for the virus and he was not prosecuted. Because the Internet is international, some have suggested that cyber crimes are also international and therefore prosecution should international and not left up to the laws of the country where the original crime took place.