It is 0530 UTC, 25 January 2003. A computer worm named Slammer has just unleashed one of the most devastating attacks on the internet ever. Within minutes, it infects nearly 90 per cent of vulnerable computers. Major net links break down, ATM machines fail and airlines have to cancel flights.

What was impressive about Slammer was the overwhelming speed of infection. There was no chance to intervene. Six years on, our defences are little better.

Scott Coull of the University of North Carolina at Chapel Hill and Boleslaw Szymanski of the Rensselaer Polytechnic Institute in Troy, New York, want to change that. They have devised a system to combat highly virulent, malicious worms by embedding defence mechanisms in key parts of the internet - akin to endowing it with an immune system.

To understand the limitations of the current strategies, imagine there's an outbreak of a biological virus. The major airports in the world decide not to let people disembark from planes flying in from an infected region but instead let the aircraft take off for other parts of the world.