Pastor-Satorras and Vespignani are condensed matter physicists working in the field of statistical physics. "We are interested in the characterization of complex systems and the study of epidemic spreading in networks was a natural extension of our work", Vespignani, who works at the International Centre for Theoretical Physics in Italy, told PhysicsWeb.

Earlier investigations of the circulation of computer viruses on the Internet were based on biological models of the spread of epidemics in the population. But Pastor-Satorras, who works at the Universitat Polytècnica de Catalunya in Spain, and Vespignani realised that there is a crucial difference in the topologies - that is, the layouts of links and 'nodes' - in the two types of network. "We analysed real data from digital epidemics and noticed that they behaved very differently from biological epidemics", says Vespignani.

The Internet is a 'scale-free' network in which just a few nodes have very many connections. Computer viruses spread very quickly through such a system because they can be transmitted simultaneously to many other sites. In contrast, the nodes in a social network have on average the same - relatively small - number of links to other nodes. An infection spreads through the population at a certain rate, but people also recover at a particular rate. This results in a critical level of the spread of the infection known as the epidemic threshold. Above the threshold the disease infects a constant proportion of the population. Below the threshold, it surfaces only in occasional pockets

"It was obvious that we needed to include the scale-free properties of the Internet in the computer virus model", says Vespignani. The pair simulated the spread of viruses using the new model and found that the epidemic threshold is completely absent from scale-free networks. This means that even a slowly spreading computer virus can proliferate over the Internet. "We hope that our work will help to devise strategies to protect computer networks from digital infections", says Vespignani.