Computer viruses now 20 years old

This week computer viruses celebrate 20 years of causing trouble and strife to all types of computer users.
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.