It’s no secret that public transportation is the epicenter of illness; from sticky subway poles covered in commuter germs, to the disaster that is public train and plane bathrooms, you should never travel without a bottle of Purel at hand. But researchers now are finding that public transportation, including airlines and commuter road travel, may not only be taking you from place to place. In their new study published in PLOS Pathogens, researchers from Emory University in Atlanta are tracking how diseases spread and travel through our methods of transportation.

It is commonly believed among the medical field that when viruses invade a population through local transmission, or person to person contact, a wave-like pattern of infection can be seen across that population. Similarly, researchers expect the viral mutation patterns to reflect that wave-like pattern, creating a positive correlation between genetic variation within mutating viruses and geographic location.

However, in a modern age where we can travel beyond the place where we were born in a variety of different ways, the spread of disease becomes a lot more complicated. Now, viruses can spread as we commute, traversing the distances we tread each day and reaching larger groups of people. To examine how genetic variation of mutating diseases may change in a more mobile society, researchers Brooke Bozick and Leslie Real, examined how air, train and car travel can explain the genetic structure of the seasonal flu.

First, researchers needed to measure the amount of people using common forms of transportation. After carefully analyzing travel networks, they found that 1.6 million people travel by air each day during the flu season, and that most U.S. states are well connected via air transportation with other states. They also found that 3.8 million people travel each day across interstate commuter networks, but that more connections of interstate ground transportation can be found in the East as opposed to the Western U.S.

Researchers then looked to different genetic variations of the flu, including the H3N3 and H1N1 subtypes of the flu, that occurred over 10 seasons; the years include 2003/2004 and 2012/2013. With this information they compared genetic distance, or the degree to which the virus has mutated from its original form, to geographic distance between certain interstate airports and certain interstate commuter stops.

They found that commuter travel was most strongly associated with the evolution of the H1N1 flu virus, suggesting that transportation systems that allow us to travel a great magnitude of distances has greater influence on the way a virus travels than the proximity of infected areas. They did not find the same instance of consistent genetic variation within the H3N2 virus.

What this all means? Our commuting habits are guiding the spread of pathogens at greater distances, while allowing the disease to mutate and ultimately, become harder to treat. While researchers found evidence that flying has a strong tendency to spread disease over far distances, it is local commuting that poses the greatest threat to regional areas.

These findings seem to be in line with previous studies, which discovered that states that border one another usually experience epidemics of certain viruses at the same time. Researchers believe that, because these epidemics usually occur between states that have high instances of commuting back and forth, the pattern is in accord with traveling. Researchers concluded in their recent press release that “the identification of these states, as well as network pathways that contribute substantially to influenza spread, is an important next step for epidemiological research.”

Source: Bozick B, Real L, et al. The Role of Human Transportation Networks in Mediating the Genetic Structure of Seasonal Influenza in the United States. PLOS Pathogens. 2015.