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World’s mailing list, which the magazine routinely rented out in the form of addressed labels. The magazine had seeded its list with names and addresses of its own staff, an ordinary practice that allows the renter to check that its clients aren’t using the list more often than agreed. It was these seeded addresses that had alerted the magazine to the diskette.

If the publication had received copies, so had some 7,000 others on the mailing list. And Jim knew that many of these would have loaded the program without reading the blue leaflet — which was, in any case, printed in type so small that it was almost unreadable. Anyone who had already run the diskette, Jim thought, could well be sitting on a time bomb.

Later that evening an increasingly anxious Mark Hamilton phoned again: ‘We’re now getting reports that this thing has been discovered in Belgium, Paris, Germany, Switzerland and Scandinavia. Oh, and Italy. Can you do anything with it?’

In fact, Jim was already working on an antidote. He had loaded the diskette on an isolated test computer in his upstairs office and had discovered that the diskette contained two very large executable files: an ‘Install’ program and an ‘AIDS’ program. Jim had previously attempted to run the AIDS file on its own but after a few seconds it aborted, displaying the message: ‘You must run the Install program before you can use the AIDS program.’

He followed the instructions, warily loading up Install. It beeped into life, the light on the hard disk flickering off and on. When the installation was finished, Jim looked at the hard disk, using specialist software to see all the files listed in the computer’s various directories. The software also allowed him to see any ‘hidden’ files, those concealed from casual inspection, generally to prevent them being deleted accidentally. There are always two hidden files on a hard disk; but now, after running the Install program, there was suddenly a whole series of them, none of them named. That seemed odd, Jim thought.

He decided to have a look at the hidden files, using another specialist program. This software went right into the heart of the files, penetrating the binary code, the building blocks of programs. It presented the contents on a vertically split screen: the left side displaying

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