In 1983 at the University of Southern California, Fred Cohen gave the first public demonstration of a class of computer program that his doctoral supervisor - Leonard Adelman, co-creator of the RSA encryption algorithm - had christened "viruses".

In his presentation, Cohen defined a computer virus as "a computer program that can affect other computer programs by modifying them in such a way as to include a (possibly evolved) copy of itself". He then went on to demonstrate an infection within a Unix directory-listing utility, and to prove that identifying and isolating computer viruses was a non-computable problem. Combating computer viruses is therefore impossible to achieve using an algorithm - an interesting result given the vast antivirus industry that sprang up in the wake of his work.

Cohen is widely regarded as the man who gave the first warning of the computer virus pandemic that we now suffer, and as the man who on 3 November 1983 wrote the first computer virus. The 20th anniversary of this tour de force has been commemorated in many publications in the past few weeks.

In fact, while Fred Cohen is indeed to be respected for his academic achievement and for the careful research that underpinned it, his was not the first demonstration of an unwelcome, self-replicating computer program infection. Credit - if that is the word - for this should instead be given to a high school student in Pittsburgh in the winter of 1981.

Rich Skrenta was a computer enthusiast and Apple II aficionado who, simply as an exercise in creativity, wrote a computer program called Elk Cloner.

This replicator spread from computer to computer on infected Apple II system disks, and every 50th time the system disk was run it would print a simple ditty on the screen concerning "Elk Cloner: the program with personality".

Elk Cloner was wildly successful, quickly infecting all the computers used by Skrenta and his friends, even infecting the computers used by the staff at his school. Skrenta himself admitted that the program was a nuisance, frequently interfering with his own use of the Apple II systems.

In the days before any form of antivirus facility, eradicating the replicator was all but impossible, and it would frequently seem to burst back into life in the closed environment within which Skrenta and his friends operated.

Elk Cloner died out not as a result of any clever antivirus strategy, but simply because the Apple II itself was superseded by other personal computer platforms in the 1980s. And because of the closed, non-networked environment at Skrenta's school, it did not get out into the wild. It is not, therefore, as well remembered as other computer problems, though it does mean that we have already missed the 20th anniversary celebrations of what is perhaps the most important phenomenon of the modern information age.

Still, we can always anticipate the 20th anniversary of the first destructive computer virus, the Brain virus of 1986. Save me a slice of the birthday cake!