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been written in Haifa, the country’s principal port and the home of its leading technical college, Technion University. At least, that is one theory. No one has proved that the virus was written in Haifa, nor has anyone ever claimed authorship.

The Jerusalem virus was a malicious joke, which as a punchline would delete any program files used on Friday the 13th. There are two Friday the 13ths in any given year; in between those dates the virus signalled its presence by displaying a little square in the lower half of the computer screen and then slowing down infected systems to an unacceptable crawl. It also contained a gremlin which, contrary to the programmer’s intentions, caused it to reinfect – or add itself to – many of the same program files. Eventually the files would grow so big that the virus would take up all of the computer’s memory.

The virus quickly acquired a fearsome reputation. Maariv, one of Israel’s leading daily newspapers, heralded its discovery with an article on 8 January 1988 that warned: ‘Don’t use your computer on Friday the 13th of May this year! On this day, the Israeli virus which is running wild will wake up from its hibernation and destroy any information found in the computer memory or on the disks.’

The report was somewhat exaggerated. It wasn’t true that Jerusalem could destroy ‘any information found in the computer memory or on the disks’ as it had been written to delete only programs that were used on any Friday the 13th. In practice, few users suffered any real damage. Most operators would delete the virus as soon as they saw the little box appear on the screen and noticed the system slow down – which was, generally, about a half hour after the virus infected a computer.

While Jerusalem may not have been as destructive as its publicity suggested, it was exceptionally virulent and spread quickly and widely. Unlike most previous viruses, Jerusalem could infect any common program file, which gave it more opportunity to travel. (By contrast, the Pakistani virus, Brain, could only infect the boot sector on specific diskettes; Lehigh could only infect one particular type of program file.)

Jerusalem’s propagation rate was phenomenal. From Israel it spread quickly to Europe and North America. A year after its discovery in Israel it had become the most common virus in the world.

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