COMPUTER viruses should be treated as a life form, Stephen Hawking, the British physicist and leading theorist on the origins of the Universe, has told an audience of computer enthusiasts in the United States.

In a speech on the nature of life, Professor Hawking said computer viruses fit standard definitions of living systems, even though they have no metabolism of their own. The computer virus exploits the metabolism of the host computer it infects, and becomes a parasite. This parasitic existence is a key characteristic of biological viruses, as is the ability to replicate only inside the cells of a living host.

'I think computer viruses should count as life,' Professor Hawking told the computer trade show in Boston.

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Computer viruses are pieces of rogue computer code. Some are relatively harmless, just as flu or glandular fever can be survived. Others can cause serious, permanent damage to computer data, just as HIV can be devastating to humans.

Computer viruses spread by attaching themselves to legitimate code, which they use as a passport into a clean computer. On a network, where computers are closely linked, infection can spread rapidly. This echoes infection by biological viruses, which usually takes place after intimate contact.

A spate of particularly nasty computer viruses recently hit the UK, traced to a writer called 'the Black Baron'. They differ from those that have gone before in that they can change the way they appear from the outside every time they infect a computer. This makes detection very difficult, but again mirrors the behaviour of some of Nature's worst viruses, which mutate as they pass from host to host.

Dr Richard Dawkins, professor of evolutionary zoology at Oxford University and author of the The Selfish Gene, said yesterday that although Professor Hawking's claim was interesting, even biological viruses are on the borderline between living and non-living things.

'Although I would agree with Stephen Hawking that computer viruses are approximately as alive as ordinary viruses, that's not very alive.' He said his criteria for identifying a living thing would be that it should have some independence, which both biological and computer viruses lack.