Working with a simple mathematical model in which chance plays a key role, researchers Steve Strogatz and Bertrand Ottino-Loofler calculated how long it would take a bacterial infection or cancer cell to take over a network of healthy cells. The distribution of incubation times in most cases, they contend, is close to “lognormal” – meaning that the logarithms of the incubation periods, rather than the incubation periods themselves, are normally distributed.