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Computer viruses (BMJ 302-66e)

John Croall
British Medical Journal, vol. 297, p.488
ISSN 0959-8138
August 1988

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Dr John Asbury points out that "a computer virus is a small piece of computer code which has been maliciously inserted on computer storage media" (23 July, p 246). Although this is true in most cases, there remains the possibility that some of these viruses have "evolved" out of random mutations ofcomputer programs.

Some computer viruses are extremely small, consisting of short lengths ofmachine code. There are many small machine code lengths built in as part of the operating systems ofcomputers to fulfil various tasks, such as reading, writing, and erasing data. Anyone who has ever used a computer will be extremely familiar with computer failures, or "crashes," when all the data in a computer's memory are scrambled and data are lost. This scrambling consists of random substitution of the computer memory locations with tiny fragments of machine code one or two bytes long. It is entirely possible, considering the number of times such events occur and the few substitutions required to mutate an existing length of machine code, into a virus (analogous to base pair substitution in DNA), that such viruses will be thrown up quite regularly. Most will be fatal mutations and will wipe themselves out. Others will be stillborn, stable but non-functioning. Even fewer will emerge as infectious viruses. According to Darwinian natural selection, the viruses that are adapted to survive hidden inside computer systems will be the most successful and may proliferate, either as a result of the initial mutation or as a result of subsequent non-fatal mutations.

Thus it is that life of a sort may have evolved in the computer systems designed by humans. The worrying thing is, as Dr Asbury suggests, that these new viruses may be very dangerous indeed, much more so than their biological counterparts. The AIDS virus may eventually kill many thousands of people. Yersinia pestis killed many millions in the Black Death. But a lively virus in the control system of an intercontinental ballistic missile may prove a greater human pathogen than any mere biological microbe.

John Croall
Public Health Laboratory,
William Harvey Hospital,
Willesborough, Ashford, Kent TN24 0LZ
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