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The current swine influenza pandemic has provided a graphic illustration of how quickly and widely a virus can spread around the world these days. From the first reports of cases in Mexico in mid-March, it has taken less than two months to spread throughout North America, much of Europe and as far south as New Zealand. Although the swine flu scare seems for now to have been contained, such events always bring to mind the Spanish flu pandemic of 1918-19, which is estimated to have killed upwards of 40 million people worldwide. Inevitably, such comparisons create widespread panic and cause untold damage to the global economy, at a time when it is already seriously weakened by the worldwide recession. Air travel, tourism, and meat imports and exports for example are all badly affected across the globe, as people cancel or delay their leisure plans and countries tread a cautious path through their food sourcing. All this is despite the fact that the travel restrictions initially recommended by the World Health Organization have now been relaxed, and that the virus is not spread through meat. It seems that, at this point, fear of the virus is causing more harm than the virus itself. So, clearly, if we could predict the spread of a disease or virus then we could take action appropriate to the level of its threat to our health - and the health of the overall economy. A mathematical model would be the obvious way forward here, but what should be equally clear is that building such a model is no simple matter. Different diseases spread in different ways, and viruses mutate, so a model of how one pandemic progressed cannot be applied to subsequent pandemics. The difficulty is that diseases and viruses don't spread in the same way as the kind of physical systems that engineers are familiar with, so there's no physical law that can be applied to them. So there are many different methods of modelling.