After SARS broke out in China in 2002, it reached 29 countries in seven months. Air travel is a major reason why such infectious diseases spread throughout the globe so quickly. And yet even with such 
examples to study, scientists have had no way to precisely predict how the next infectious disease might spread through the nexus of world air terminals—until now.

In 2010 MIT engineer Ruben Juanes set out to model the movement of a pathogen from a single site of departure to junctions worldwide. If he could predict the flow of disease from a given airport and rank the most contagious ones, government officials could more effectively predict outbreaks and issue lifesaving warnings and vaccines. So Juanes and his team used a computer simulation to seed 40 major U.S. airports with virtual infected travelers. Then they mimicked the individual itineraries of millions of real passengers to model how people move through the system. The travel data included flights, wait times between flights, number of connections to international hubs, flight duration, and length of stay at destinations.

JFK International in New York—one of the world’s most heavily trafficked airports—emerged as the biggest culprit in disease spread. Honolulu, despite having just 40 percent of JFK’s traffic, came in third because of its many long-distance flights. The biggest surprise: The number of passengers per day did not directly correlate to contagion risk.

1 New York (JFK)  JFK has over 1,000 daily flights, connecting some 200 airports in more than 60 countries. The number of international connections allows passengers here to come in contact with individuals from many points of origin, dramatically increasing the risk that infected travelers could pass disease to uninfected populations worldwide.

2 Los Angeles (LAX)  Los Angeles International has lots of traffic, 
supporting more than 1,400 flights a day 
and connecting some 
55 countries.

3 Honolulu (HNL)  Honolulu International gets only two-fifths of JFK’s traffic, yet it poses a major risk because it has a high proportion of long-distance flights, links to well-connected airports, and a geographic location that encourages an equal diffusion of travelers 
going east and west.

4 San Francisco (SFO)

5 Newark, NJ (EWR)

6 Chicago (ORD)

7 Washington, D.C., Dulles (IAD)

8 Atlanta (ATL)  While Hartsfield-Jackson Atlanta International ranks first in the world for traffic (roughly 2,600 flights a day), most of these flights are regional, leaving the airport relatively unconnected to far-off locations that would boost its ability to spread infection.

Individual Itinerary
The highlighted route shows two trips, one from San Francisco to New Orleans and back, the other following the same route with a layover in Chicago. This is a typical itinerary: The traveler moves to and from a home base in a major city, either through direct flights or incorporating stopovers. Passengers remain at a destination an average of four days—a crucial data point, since a disease’s transmission rate depends on the duration of exposure.