In addition to the connectivity provided by new steam, rail, and road lines, the construction of railroads and other large infrastructure projects led to cultural changes that also had an important impact. Large groups of men were conscripted, often forcefully, to build railroads. Moore and his colleagues note that the labor camps were populated mostly by men, a condition that dramatically favors transmission of sexually transmitted viruses like HIV. Together, the shipping and rail routes and the factors surrounding their construction must have played a role in the early transmission and spread of HIV.
As dramatic as the road, rail, and shipping revolutions were for the transmission of microbes, an entirely new form of transport would add another layer of speed. On December 17, 1903, in Kitty Hawk, North Carolina, a site chosen for its regular breeze and soft sandy landing areas, the Wright Brothers made the first sustained, controlled, and powered flight. Some fifty years later the first commercial jet flew between London and Johannesburg. By the 1960s, the age of jet travel was here to stay.
Airplanes link populations in an immediate way, which allows the transmission of microbes to occur even more quickly. Microbes differ from each other in terms of their latent period, the period of time between when an individual is exposed and when they become infectious or capable of transmitting the agent to others. Almost no microbes that we know of have latent periods of less than a day or so, but many have latent periods of a week or more. The immediacy of air travel means that even microbes with very short latent periods can spread effectively. In contrast, if a person infected with an agent that had a very short latent period were to board a ship, unless the ship had hundreds of individuals the virus could infect, it would go extinct before the ship made land.
Commercial air flights alter in fundamental ways how epidemic disease spreads. In a fascinating paper from 2006, my colleagues John Brownstein and Clark Freifeld of Harvard, one of the new academic breed of digital epidemiologists, found creative ways to use existing data to show just how much impact air travel has on the spread of influenza. John and his colleagues analyzed seasonal influenza data from 1996 to 2005 and compared it with patterns of air travel. They found that the volume of domestic air travel predicts the rate of spread of influenza in the United States. Interestingly, the November travel peak around Thanksgiving appears to be of particular importance. International travel also plays a vital role. When the number of international travelers is lower, the peak of the influenza season comes later—because when there are fewer travelers, it takes longer for the virus to spread. Perhaps most strikingly the researchers were able to see the impact of the terrorist attacks of September 2001 on influenza. The travel ban led to a delayed influenza season. The striking effect was not seen in France, which did not enact the ban, providing an excellent control.
During the past few centuries the ease of movement has increased dramatically throughout the world. The rail, road, sea, and air revolutions have all permitted humans and animals to move more quickly and efficiently both within continents as well as between them. The transportation revolution has created interconnectivity unprecedented in the history of life on our planet. It is estimated that we now have over fifty thousand airports, twenty million miles of roads, seven hundred thousand miles of train tracks, and hundreds of thousands of ships and boats in the oceans at all times.
The connectivity revolution we've experienced has fundamentally changed the ways that animal and human microbes move around our planet. It has radically increased the speed at which microbes can travel. It has brought populations together, allowing agents that couldn't previously sustain themselves with low population numbers to flourish.
Excerpted from The Viral Storm: The Dawn of a New Pandemic Age by Nathan Wolfe, published October 11th by Times Books, an imprint of Henry Holt and Company, LLC. Copyright © 2011 by Nathan Wolfe. All rights reserved.
For more about stopping deadly viruses from spreading, read Nathan Wolfe's 2009 feature "How to Prevent the Next Pandemic" in Scientific American.