The 20th anniversary of the first PC virus falls this month.
The first PC virus travelled around on floppy disks
It was during the opening weeks of 1986 that the first PC virus, called Brain, was discovered in the wild.
Though it achieved fame because it was the first of its type, the virus was not widespread as it could only travel by hitching a ride on floppy disks swapped between users.
Now 20 years after they first appeared there are more than 150,000 malicious programs in existence.
The origins of the Brain virus are disputed. It is thought to have been created by a Pakistani software firm to help protect the software it created and sold.
The virus was discovered in January 1986 but may have been written some time before that as it used a relatively slow method to travel.
Brain was known as a "boot-sector" virus because of the area on a floppy disk it hid on. By concealing itself in this region, the virus could ensure that it would be installed every time that floppy disk was used on another computer. The Brain virus is now extinct.
Although Brain was the first PC virus, it was not the first malicious computer program. That honour goes to the Elk Cloner virus written by Richard Skrenta which infected Apple II machines.
The term computer virus dates from 1984 and was coined by US computer scientist Fred Cohen.
Since Brain first appeared, the number of viruses and other malicious programs in circulation has grown enormously. Microsoft's Windows operating system is the favourite target of virus writers.
The growth of local computer networks, e-mail and the internet has fuelled this rise and now it can take only hours for a new virus to appear and infect thousands. There are also many different types of viruses that use varying attack types to infect machines.
The reasons people write viruses has also changed.
"The most significant change has been the evolution of virus writing hobbyists into criminally operated gangs bent on financial gain," said Mikko Hypponen, chief research officer at Finnish anti-virus firm F-Secure.
This week the FBI released figures which suggest that 84% of US businesses were attacked by viruses, spyware and other malicious programs in 2005.
On average, dealing with the attack cost US businesses $24,000, said the FBI.