Each of the major transportation advances would alter connectivity between populations, and each would have their own impact on the spread of new microbes. The exclusivity of ships as a means for long-distance transport would not hold out forever. The use of roads, rail, and air provided new connections and routes for the movement of humans and animals as well as their microbes. For microbes, the transportation revolution was really a connectivity revolution. These technologies created links that forever changed the nature of human infectious diseases, including, critically, how efficiently they spread.
The use of roads of some sort or another is an ancient practice, far predating the use of water as a medium for transportation. Chimpanzees and bonobos both create and use forest trails to help them move through their territories. I learned this firsthand while studying wild chimpanzees in the Kibale Forest National Park in southwest Uganda. Richard Wrangham, the Harvard professor who introduced me to this work, used these trails to help observe chimpanzees.
Wrangham had done his doctoral work at the Gombe Stream site in Tanzania that Jane Goodall established. He'd critiqued some of the findings from Gombe because the chimpanzees there were habituated by provisioning—to get the wild chimpanzees comfortable with human researchers, the animals were fed large amounts of banana and sugarcane. Wrangham felt that provisioning changed some of the subtle chimpanzee behaviors, so when he started his own site in Kibale, he habituated the animals the hard way—by having his teams follow them until the apes effectively gave up and no longer ran away. He did this by essentially enhancing and extending the natural pathways that they moved along.
The art of actual road building began in earnest around five to six thousand years ago when cultures throughout the Old World started using stone, logs, and later brick to enable the movement of people, animals, and cargo. The first modern roads followed in the late eighteenth and nineteenth centuries in France and the United Kingdom. These roads used multiple layers, drainage, and eventually cement to make permanent structures permitting regular movement throughout the year.
The rate at which modern roads have spread throughout the world has not been consistent, of course. Some regions in Europe and North America have roads reaching most human populations, while some regions where I work in central Africa have virtually no road access. Clearly, as roads enter into new regions, they bring both positive and negative effects. They are among the top priorities for many rural communities since they provide access to markets and health care, but from the perspective of global disease control, they are double-edged swords.
HIV is among the most notable example of the impact that road proliferation has had on the movement of microbes. In a series of fascinating studies, the HIV geneticist Francine McCutchan, whose lab I worked in at Walter Reed Army Institute of Research (WRAIR), and her colleagues at the Rakai and Mbeya sites in east Africa have examined the role that roads have played in the spread of HIV, demonstrating that proximity to roads increases a person's risk of acquiring HIV. As people have more access to roads, they have a higher chance of getting infected because roads spread people, and people spread HIV. Other than sex workers, the highest occupational risk for acquiring HIV in sub-Saharan Africa is being a truck driver. McCutchan and her colleagues have shown that the genetic complexity of HIV is greater among individuals who have increased access to roads. Roads provide the mechanism for different types of HIV to encounter one another, in a single coinfected individual, and swap genetic information. But roads do more than just help established viruses spread. Roads and other forms of transport can also help to ignite pandemics.
One of the most stubbornly lingering public misconceptions is that we don't know how HIV originated. In fact, our understanding of the origins of HIV is more advanced than our understanding of the origins of probably any other major human virus. As we saw in chapter 2, the pandemic form of HIV is a chimpanzee virus that crossed into humans. There is no debate within the scientific community on this point. The cumulative evidence with regard to how it originally entered into humans is also increasingly unequivocal. It was almost certainly through contact with chimpanzee blood during the hunting and butchering of chimpanzees. We'll delve further into this in chapter 9 when we discuss the work my colleagues and I have done with central African hunters.
Perhaps the only lingering debate about HIV origins is how it originally spread from the first infected hunter and why it took so long for the medical community to discover it. The earliest historical HIV samples date from 1959 and 1960, twenty years before AIDS was even recognized as a disease. In an amazing piece of viral detective work, evolutionary virologist Michael Worobey and his colleagues managed to analyze a virus from a specimen of lymph node from a woman in Leopoldville, Congo (now Kinshasa, DRC).
The lymph node had been embedded in wax for over forty-five years. By comparing the genetic sequence of the virus they found in the specimen with other strains from humans and chimpanzees, they were able to attach rough dates for the first ancestor of the human virus. While the genetic techniques they used cannot pinpoint dates closer than a few decades, they concluded that the virus split from the lineage sometime around 1900 and certainly before 1930. They also concluded that by the time that the woman in Leopoldville became infected with HIV in 1959 there was already a significant amount of genetic diversity of HIV in Kinshasa, suggesting that the epidemic had already established itself there.
The fact that HIV goes back to 1959, let alone 1900, provides some serious challenges to the medical community. One of the central questions is this: if it was in human populations in the early twentieth century and already constituted at least a localized epidemic in Kinshasa by 1959, why did it take us until 1980 to identify the epidemic? Another key question is what special conditions were present that permitted the virus to start taking off in the middle of the twentieth century?
A number of changes occurred in francophone central Africa, the region where HIV-1 originated, leading up to the period in the 1950s when those first precious samples were taken. The anthropologist Jim Moore and his colleagues at the University of California, San Diego, put together some of the key events in a 2000 paper, the majority of which focused on how easier means of travel influenced virus proliferation. In 1892 steamship service began from Kinshasa to Kisangani in the very heart of the central African forest. The steamship service connected populations that had been largely separated, creating the potential for viruses that previously might have gone extinct in local isolated populations to reach the growing urban centers. In addition, the French initiated the construction of railroads, which, like shipping and road lines, connect populations. This produced another mechanism for viruses to spread from remote regions to urban centers, effectively providing a larger population size of hosts for a spreading virus.