Human movers, including but not limited to planes, trains, and automobiles, always carry something extra, a bit of viral baggage. Now, epidemiologists at Emory University are demonstrating how microscopic stowaways can use transportation infrastructure to move with and around human hosts. The results serve as a reminder that large transport projects, particularly those underway in developing countries, present economic, personal, and viral opportunity.

The Emory study’s authors charted viral spread by examining the disperasal of seasonal flu from 2003 to 2013 then comparing trends to data on U.S. transportation networks. They found that highly connected states share similarly timed epidemic peaks, and that ground transportation played a more important role the transmission of flu across networks. Transportation systems in general had a greater influence on a virus’ spread than did the proximity of infected areas.

In other words, convenience correlates very strongly with transmission.

Two specific cases that were compared to one another were the transmission routes from H1N1 (the subtype responsible for the 1918 Spanish Flu pandemic, and the 2009 Swine Flu pandemic), and H3N2. Genetic variation in H1N1 were found to be strongly associated with commuter travel, proving that human commuting habits play a key role in allowing viruses to mutate faster. These findings more-or-less confirm what was already suspected about the travel habits of flu in the US, but the scientists were able to highlight specific interstate networks that affect one-another during flu season, which is rather remarkable and useful if you’re planning a road trip.

An image of the U.S. Interstate Commuter Network that shows the number of people traveling daily between states for work.

The U.S., like many developed countries, has a strong surveillance system in place for detecting rare diseases and outbreaks, and it makes it fairly easy for health officials to track how infectious pathogens might spread. But developing countries rarely have that kind of network in place. And many are embarking on massive projects with tremendous potential to change the human and viral landscapes.

These may be the next great viral throughways.

Trans-Amazonian Railway

When construction of the Trans-Amazonian Highway — which runs across Brazil to eastern Peru — began in the 1970s, health officials noticed a spike in reports of locals coming down with various parasitic illnesses like malaria, leishmaniasis, Chagas disease, and toxoplasmosis (a.k.a. the disease brought upon by the brain-eating ameoba). The construction and deforestation seemed to have made vectors like mosquitos and kissing bugs more aggressive, and they began biting humans more frequently. A $10-billion railway project across several different countries will likely bring about another wave of parasitic disease transmission.

Marmaray Project

For centuries, Istanbul has played a crucial role as a link between Europe and Asia. The Marmaray Project is a 47.2 mile undersea rail tunnel running under the Bosphorus strait that aims to make this solidify this connection even further. But after it’s completed, each region may get more than they bargain for.

Because of its geographical location, Turkey has always played host to different diseases, including tuberculosis, West Nile virus, and hepatitis. Recently, there have been increased reports of antibiotic-resistant bacterial infections. When it’s finished, the Marmaray rail could be a gateway that creates faster transmission of common diseases in the East make their way West — and vice versa.

LAPSSET Project

The acronym stands for the Lamu Port-South Sudan-Ethiopia Transport Corridor, and is meant to include plans for an oil refinery, pipelines, ground and air transportation hubs, and a port for oil tankers. The African nations of Kenya, Ethiopia, and South Sudan hope this massive, $25 billion infrastructure project — currently in the planning stages — will be a huge boon in the development of all three countries (pending pirate developments).

But while the ongoing ebola epidemic has been largely confined to the western half of the content, East Africa is no stranger to major disease outbreaks, having most recently dealt with epidemics in cholera, malaria, and hemorrhagic fever. The region already deals with a cyclic meningitis outbreak that affects countries along the “meningitis belt,” including Kenya and Ethiopia. All infrastructure projects run the risk of exacerbating the control of these outbreaks, but a large economic landmark like the LAPSSET Project would attract businesspeople from all over the world — all of whom could potentially bring a deadly infection back to their home country.