COMPUTER viruses, the self-replicating programs that infect workstations and cripple networks, are a hazard to business. The “I love you” worm, which wreaked havoc on May 4th 2000, spread by sending a copy of itself in an e-mail to everyone in every address book it encountered. It took just a few mugs in each company to open the attached “love letter” to generate thousands upon thousands of messages that overwhelmed systems round the world. The cost to business was estimated to be $10 billion.

The “I love you” worm was exceptional, but run-of-the-mill viruses are a constant nuisance. However, a paper published in this month's Nature Physics, by Eran Shir of Tel-Aviv University in Israel and his colleagues, has suggested a new way of dealing with them.

A virus spreads by infecting its host, reproducing itself and moving on. Firewalls and other sorts of anti-virus software—computer programs that attempt to identify, thwart and eliminate computer viruses and other malicious bits of code such as spyware—are one line of defence. These programs search files for known viruses and monitor what is running for suspicious behaviour. If a known virus is encountered, it can be stopped.

But new viruses are continually being created. As soon as a new virus is detected, a second line of defence would be to pass information about it between computers quickly, before they become infected. And that is what Mr Shir is proposing. Moreover, the researchers believe it takes a virus to stop a virus. They would like to propagate the vaccine as an epidemic.

As in the biological world, a big problem with vaccines is that they cannot be created until the virus they seek to stop spreading has emerged. Vaccines are thus always one step behind. What Mr Shir and his colleagues outline is a plan to accelerate the spread of a vaccine in a way that allows it to overtake the virus.

Their strategy uses a series of “honey pots”. These pieces of software attract viruses and are employed by anti-virus firms to pick new threats up quickly. The researchers' idea is to embed these honey pots within a network so that any virus spreading through the network would be likely to reach them promptly. The honey pots would then be linked to each other in a separate, overlying network that allowed only vaccines to pass across it.

This separate network would act as a by-pass, enabling the vaccine to overtake the virus and reach uninfected computers first. A honey pot would automatically identify a new virus, extract its signature and pass this information—the vaccine—to other honey pots using the overlying network. The honey pots would share this information with other computers on the network. All uninfected computers would then be able to recognise the new virus as dangerous, and refuse it entry.

The strategy would become more effective as the size of the network grew. For example, for a network of 50,000 computers, if 0.4% of them were honey pots, some 5% of computers would become infected by a virus before they were immunised. If there were 200,000 computers, the infection rate would fall to about 1%. For 200m computers (about the number now in America), less than a thousandth of a per cent would be infected. The savings to business could be great indeed.