This week computer viruses celebrate 20 years of causing trouble and strife to all types of computer users.
Now most viruses arrive via e-mail
US student Fred Cohen was behind the first documented virus that was created as an experiment in computer security.
Now there are almost 60,000 viruses in existence and they have gone from being a nuisance to a permanent menace.
Virus writers have adapted to new technology as it has emerged and the most virulent programs use the net to find new victims and cause havoc.
Mr Cohen created his first virus when studying for a PhD at the University of Southern California.
Others had written about the potential for creating pernicious programs but Mr Cohen was the first to demonstrate a working example.
In the paper describing his work he defined a virus as "a program that can 'infect' other programs by modifying them to include a ... version of itself".
Mr Cohen added his virus to a graphics program called VD that was written for a make of mini-computer called a Vax.
The virus hid inside VD and used the permissions users had to look at other parts of the Vax computer to spread around the system.
In all the tests carried out by Mr Cohen the virus managed to grab the right to reach any part of the system in less than an hour. The fastest time was five minutes.
The creation of the virus gave rise to such consternation that other tests were banned, but Mr Cohen did manage to demonstrate a similar virus working on other computer systems.
Viruses used to travel via floppy
In the paper Mr Cohen prophetically wrote: "they can spread through computer networks in the same way as they spread through computers, and thus present a widespread and fairly immediate threat to many current systems."
Soon after this pioneering work viruses written for the IBM personal computer, which had only just been created, started to appear.
The first of these is widely acknowledged to be the "Brain" virus that emerged in 1986 from Pakistan and was, apparently, written to help its creators monitor piracy of their computer programs.
The emergence of Brain kicked off lots of other viruses such as Lehigh, Jerusalem, Cascade and Miami.
All these were aimed at PC users and travelled in floppy disks that passed around as the programs they held were used on different computers. Though they were a nuisance to those they caught out they were something of a rarity.
Efforts to spot and stop viruses forced creators of the malicious programs to find ways of hiding their creations sometimes by making them change form to avoid detection.
In 1992 the Michelangelo virus, that was due to strike on 6 March, caught the media's attention but the chaos it was predicted to cause never materialised.
As Windows emerged virus writers began targeting the new operating system.
The Love Bug tricked many people into opening it
This led to an explosion in so-called "macro" viruses that exploited the crude utility writing program in Microsoft Word.
These viruses were much more widespread because people shared far more documents than they did the programs that early viruses piggy-backed upon.
As Windows has emerged in successive versions, virus writers have kept pace with the new technology.
The Melissa virus that struck in March 1999 marked a new trend as it combined a macro virus with one that plundered the address book of Microsoft Outlook to e-mail itself to new victims.
The success of Melissa was largely due to the fact that the net was becoming increasingly popular and the most successful viruses of recent times have exploited weaknesses in e-mail programs or net connected PCs.
Almost every year since 2000 has seen the unleashing of a virulent program that uses the net to travel.
The Love Bug struck in 2000 and was followed by the Nimda and Code Red viruses that swamped net connections.
More recently we have had Sobig, Palyh, Slammer and MSBlast viruses that have spread further and caused more havoc than early virus writers could have ever imagined.