How a Local Disease Becomes a Global Epidemic

Researchers tracking the global path of illnesses such as Ebola say that the frequency of long distance travel, not a high concentration of sufferers in one area, is a crucial factor in changing an outbreak into a worldwide epidemic.

The biophysicists, from the University of California, Berkeley, and Stanford University, found that if fewer people were traveling long distances, the illness was less liable to become an epidemic. Instead, it was likelier to spread in a slow wave, the way other epidemics such as the Black Death have done throughout history.

Using a very simple model involving a petri dish, Oskar Hallatschek, assistant professor of physics at Berkeley, and co-author Daniel Fisher of Stanford, found that if the chance of “long-distance jumping” is above a certain level – as it is today with air travel – the illness can spread like wildfire.

“We clearly show that one of the key factors that controls the spread of infection is how common long-range jumps are in the dispersal of a disease,” said Hallatschek, who is the William H. McAdams Chair in physics. “And what matters most are the rare cases of extremely long jumps.”

When the “jumps” are somewhat less common, the disease spreads rapidly, as in the case of the respiratory condition SARS. If jumps are neither very rare nor very common, they produce some “satellite outbreaks” but ultimately spread more slowly than was the case with SARS.

Hallatschek said the new model will help epidemiologists better understand not only the spread of disease but also cancer metastases and genetic mutations.

The study appeared in the journal Proceedings of the National Academy of Sciences.