Any changes that create new intersections between microbes and people pave the way for disease-causing agents to enter our species. One such change that has put us at risk is the global human population explosion—from about 1.6 billion people in 1900 to almost 7.3 billion in 2016. Humans have cleared forests for agriculture, settlement, and urban expansion, leading to closer contact with environments that may harbor diverse wildlife species and the pathogens they carry. The massive expansion of roads and human settlements through much of the world’s developing tropical regions has also created transition zones filled with opportunities for contact with potential disease-causing agents.

Human travel and commerce have introduced other risks. About 2.4 million passengers, each a potential carrier of infection, travel daily by aircraft to international destinations. International commerce, especially in foodstuffs, adds to the global traffic of disease-causing microbes. Because the transit times of people and goods are often shorter than the incubation periods of infection, carriers of a disease can arrive at their destination before the infection they harbor is detectable. International trade and travel are associated with the spread of such infectious agents as HIV, severe acute respiratory syndrome (SARS), Middle East respiratory syndrome coronavirus (MERS-CoV), drug-resistant malaria, chikungunya, and Zika. In 2010, after more than an 80-year absence, cholera was believed to have been brought back to Haiti by humanitarian workers from Nepal who came to help after the earthquake. In sum, the widening and deepening of international trade are accelerating the rate at which new zoonotic (non-human animal) diseases are emerging and being transmitted as well as leading to changes in the range of their corresponding animal vectors.

 

Changes in human demographics and behavior are also linked with the emergence and spread of infections such as HIV and hepatitis C, which are transmitted through sexual activity and intravenous drug use. More broad-scale changes that raise the risk of infectious disease include the breakdown of public health systems, poverty, war, and famine. Because 60 percent of infectious diseases are of zoonotic origin, concepts such as “One Health,” which link human, animal, and wildlife health issues, are becoming more important. This approach helps to improve surveillance across species and to identify viruses that spill over to humans more quickly than they had previously. One example of these human–animal interactions is intensive farming practices, which resulted in closer human contact with animals. This, in turn, led to the emergence of variant Creutzfeldt-Jakob disease (from mad cow disease), Nipah virus (via pigs), and avian influenza (via waterbirds). Additionally, growing rates of bushmeat hunting, which also brings humans into closer contact with wild animals, may have contributed to outbreaks of Ebola and AIDS.