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the files in computer code, the right in ordinary text. Jim went through them page by page. He discovered that the hidden files contained a counter, which kept track of the number of times a computer was turned on. After ninety such boot-ups the hidden files would spring to life and attack the computer's hard disk, encrypting working files and hiding programs.1 Without access to programs and data, the system would be unusable.

The diskette was, Jim realized, a huge trojan horse, a malicious piece of software that entered a system in the guise of something useful, then unleashed its payload. In this case the 'useful' component was the 'AIDS information' file; the payload was the scrambling of the hard disk.

It was curious, Jim thought, that the damned thing had been written to behave almost like the real AIDS virus. The technological bug was opportunistic, just like its biological counterpart. Both slowly, insidiously, infected the victim's immune systems; both were patient; both were ultimately fatal to their hosts. Whoever wrote the program must be interested in AIDS, though perhaps he didn't know too much about the subject. Switching to the AIDS information file, Jim read through the material it offered. It described itself as 'An interactive program for health education on the disease AIDS . . . The health information provided could save your life . . . Please share this program diskette with other people so that they can benefit from it, too . . .'

The program offered 'up-to-date information about you and the things you can do to reduce your risk of future infection, based on the details of your own lifestyle and history.' It required a user to answer thirty-eight questions – sex, age, number of sexual partners since 1980, medical history, sexual behaviour and so on – and according to the user's answers it provided 'confidential advice', some of which was rather eccentric: 'Scientific studies show that you cannot catch AIDS from insects' and 'AIDS can be prevented by avoiding the virus' were two of the less helpful confidences. Others included: 'Danger Warning: Reduce the number of your sex partners now!' 'You are advised that your risk of contracting AIDS is so large that it goes off the chart of probabilities.' 'Buy condoms today at a pharmacy when you leave your office.' 'Insist that your sex partner be

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