In the 14th century, the bubonic plague swept through Europe — an outbreak known to history as the "Black Death." This epidemic, which cost Europe a third of its population, first hit China in the 1330s as the Mongol empire expanded across central Asia. From there, it spread by caravan routes to the Crimea and the Mediterranean.

Accelerating pace of microbes

As the 21st century begins, the process of globalization is dramatically accelerating the pace at which microbes travel the globe. The rapid growth in international air travel is a particularly potent force for global disease dissemination, as air travel makes it possible for people to reach the other side of the world in far less time than the incubation period for many ailments.

Over the past two decades, more than 30 infectious diseases have been identified in humans for the first time.

At the same time, adventure tourism and other pursuits are drawing people to ever more remote locations, increasing the chance that microbes will be introduced to vulnerable populations.

Environmental degradation is another powerful contributor to many of today's most pressing global health threats. The World Health Organization (WHO) estimates that nearly a quarter of the global burden of disease and injury is related to environmental disruption and decline.

Killer environment

For certain diseases, the environmental contribution is far greater. Some 90% of diarrheal diseases such as cholera, which kill 3 million people a year altogether, result from contaminated water. And 90% of the 1.5 to 2.7 million deaths caused by malaria annually are linked to underlying environmental disruptions such as the colonization of rainforests and the construction of large open-water irrigation schemes, both of which increase human exposure to disease-carrying mosquitoes.

The rise of infectious diseases

When globalization and environmental decline join forces, the health implications can be staggering. Numerous urgent global health challenges loom. Over the past two decades, more than 30 infectious diseases have been identified in humans for the first time — including AIDS, Ebola, hantavirus and hepatitis C and E.

The process of globalization has radically accelerated the pace at which microbes travel around the globe.

The United States, for example, has never been the same since October 1999, when health experts confirmed that at least five people in New York City and surrounding areas died from a new strain of the African West Nile virus.

Until then, the rare mosquito-borne disease had never been seen before in the western hemisphere.

Traveling microbes

The experts attribute the emergence of the disease to the steady rise in international trade and travel, concluding that the disease was transmitted either by a smuggled exotic bird or by an infected human who carried it into the country from abroad.

Although the global interdependence of human and ecological health is creating frightening vulnerabilities, it is also generating an imperative for countries of the North and South to work together to confront shared perils.

Transcontinental epidemics

Faced with raging transcontinental epidemics of cholera and plague in the mid-19th century, European governments convened 12 International Sanitary Conferences between 1851 and the start of World War I in 1914. These meetings helped forge a series of international health agreements covering issues such as quarantines, trade restrictions, and procedures for disease notification and inspection.

Air travel makes it possible for people to reach the other side of the world in far less time than the incubation period for many ailments.

In 1946, these and later efforts culminated in the creation of the World Health Organization, which has had a number of important successes in its first half-century, perhaps most notably the virtual eradication of smallpox in 1977.

This system provides a firm foundation on which to build the new biological controls needed to protect people and ecosystems from the introduction of disruptive exotic species and diseases. Although economic globalization dominated headlines at the close of the twentieth century, ecological integration may pose even greater challenges for international cooperation in the decades ahead.