Influenza viruses give rise to epidemics virtually every year and they occasionally also lead to pandemics (i.e. epidemics on a global scale). Unlike the epidemics, influenza pandemics are, to a great extent, unpredictable. Thus although it is generally expected that another influenza pandemic will occur, we cannot predict when this will be. Furthermore, if a pandemic arises abroad it is difficult to predict how long it will be before it reaches our own country. And once it has arrived here, we can only make a partial estimate of how many people will fall ill, which population groups are at greater risk and which individuals, being ill, run a greater risk of complications...