It is 20 years since the release of the world's first PC virus, according to antivirus firm F-Secure.

'Brain' was a boot sector virus created by two brothers, Basit and Amjad Farooq Alvi, to protect a game they had written.

The exact day of creation is open to question, however, and some experts have suggested that the 'Ashar' virus pre-dated 'Brain'.

These viruses were spread via floppy disc and could be avoided fairly simply by making the disc read-only. Since then virus propagation has become significantly easier, and transmission rates have skyrocketed since the birth of the internet.

"It will be interesting to see what kind of viruses we will be talking about in another 20 years - computer viruses infecting household appliances perhaps?" said Mikko Hyppönen, chief research officer at F-Secure.

"The most significant change has been the evolution of virus writing hobbyists into criminally operated gangs bent on financial gain. This trend shows no signs of stopping."