How Will Disease Spread?

Cell phone study offers glimpse into how people -- and illnesses -- move.

June 5, 2008 — -- In a study designed to track how large-scale disease might spread, researchers have made a not-so-surprising discovery: Most people are slaves to routine and tend to stay close to home.

Over the course of six months, three Northeastern University researchers tracked cell phone users in Europe in search of information on human travel patterns that could be used in disease epidemic prevention and urban planning. Epidemiologists have looked to human mobility studies as a way to predict how an epidemic might spread as a population moves around.

"Most individuals spend most of their time in a very restricted area, while some individuals spend most of their time over a very large area," said Cesar Hidalgo, physics researcher at the university's Center for Complex Network Research and co-author of the study that was published in the journal Nature Wednesday.

With the help of a European cell phone company in an unnamed country, Hidalgo, along with co-authors Marta Gonzalez and Albert-Laszlo Barabasi, followed the cell phone usage patterns of 100,000 anonymous users over six months by monitoring the cell towers that route calls and text messages. Researchers estimated the approximate location of the user according to which cell phone tower handled the call.

Additionally, for one week, the researchers followed a group of 206 cell phone users equipped with programs that allowed their location to be recorded every two hours by their cell phone company. The numbers were disguised with what Hidalgo called "ugly code" – a long series of letters and numbers unique to each individual.

The smaller group was studied "to check [that] the results we were getting were accurate," Hidalgo said.

According to Hidalgo, over the six months, 73 percent of people spent most of their time within the same 10-mile radius. Between 2 and 3 percent of people moved regularly over several hundred miles.

Researchers could also reliably predict which users would be in the high-traveling group or the stay-near-home group after viewing their activity for four or five days.

This isn't the first time that researchers have examined how disease might spread. Last fall, a researcher at Rutgers University studied how a fictional epidemic that broke out in the virtual world World of Warcraft could be used to predict how a pandemic might spread in the real world.

In the game, a disease called Corrupted Blood broke out among the players, and it was spread entirely by human contact.

"There are actually parallels between what would happen in the real world and what happened in this game," said Nina Fefferman, professor of applied mathematics at Rutgers University, who published a study on the topic last August.

"This gives us the opportunity in the future to tailor infections inthe virtual world to see what would happen in the real world," she told at the time.

The most well-known mobility study is probably Where's George? a site where people can track the movement of US currency by serial number as it makes its way across the country.

Hidalgo envisions turning on the television for disease forecasts, according to human mobility studies.

"The thing is when you have some catastrophic event you want to have some way to react. This information is going to help you react in the best way that you can," he said. "I don't think it's going to happen in the short-term. ... Data is going to be the crucial ingredient to make that happen. It would be like trying to predict the weather without weather stations. You need the data."

Study Raises Privacy Issues

In addition to offering a window into how people travel, the study, the first of its kind done by the university, raised several privacy issues.

First, the cell phone users were being tracked without their knowledge. In the United States, that type of monitoring without a customer's permission is illegal, an FCC spokesman told The Associated Press.

Hidalgo, however, defends the use of the data, comparing it to political exit polling or data collected on the development of certain diseases, such as HIV cases or tuberculosis.

"We did have [privacy] concerns, and obviously, we went through great lengths to make sure the data was anonymized and secure," Hidalgo said.

"At a global level there is still a need for the information. That doesn't [mean we] need to sacrifice individual privacy. … We're scientists and we're trying to understand things. We're trying toimprove the way that the world functions."

Dan Childs contributed to this report.