Web Exclusives: Diseases
How Contagious Diseases Spread through Communities
Most of us know that colds can spread like wildfire, especially through schools. New research using human networking theory may give a clearer picture of just how the common cold, influenza, whooping cough, SARS and other infectious diseases spread through a closed group of people—and even populations at large.
A lot of these diseases spread when you cough or sneeze on someone nearby, spreading germs through droplets in the air. Yet we know very little about the dynamics of these so-called close proximity interactions. To get a better sense, Stanford University scientists developed a new technique to count the number of possible disease-spreading events that occur in a typical day at an American high school.
"Theoretically, we know that every day people come into contact with many other people, that interactions vary in length and that each contact is an opportunity for a disease to spread," says biologist Marcel Salathé, now at Penn State University. "But it's not like you can take a poll and ask people, 'How many different people have breathed on you today, and for how long?' We knew we had to figure out the number of person-to-person contacts systematically."
To do this, Salathé and his team used a population of 788 high school students, teachers and staff as a model for a closed group of people. They gave each person a matchbox-sized sensor called a mote to wear around their necks. Like a cell phone, each mote had its own unique tracking number. It also was programmed to send and receive radio signals at 20-second intervals to record the presence of nearby motes as the volunteers sat in class, walked the halls and chatted with others.
"You need to be in close proximity to be affected by droplet transmission, and the motes were ideal for [capturing] that," says Marcus Feldman, a population theoretician and Salathé's postdoctoral mentor at Stanford.
At the end of the day, the researchers collected the motes and recorded the number and length of mote-to-mote interactions. They counted a total of 762,868 close proximity events. In addition, they found peaks of interactions at times between classes, when mote-wearing volunteers were physically closer to one another as they walked to the next class. Most people experienced a fairly high number of person-to-person interactions, but with very little variation among individuals.
"While there may have been popular kids with a few more interaction events, for the most part, everyone had about the same high level of interaction," says Salathé. He adds that while schools may indeed be "hot beds" for colds and the flu, individual students don't seem to vary with regard to exposure risk due to their contact patterns.
Data from the motes also confirmed an important social networking theory: Contact events are not random because many "closed triangles" exist within a community. In other words, if you meet up with Susie and Susie meets up with Charlie, you're likely to meet up with Charlie, too.
"Real data illustrating these triangles provide just one more piece of information to help us track how a disease actually spreads," says Salathé.
It's important to remember that the data represent one day at one school and may differ between settings. But Feldman says this kind of experiment can be used with any human population—that volunteers, of course—and even with animal populations that may experience new infectious diseases.