"There are more than two million people crossing international borders every single day," explains Nils Daulaire, M.D., a DMS Overseer who used to be a member of the faculty and is now the president of the Global Health Council, a worldwide advocacy group for health initiatives. "It's no longer a hermetically sealed world."

One way to understand the problem is to look at the explosion of computer viruses. New Internet viruses originate all over the globe. They can zip through cyberspace and infect millions of computers before anyone even figures out what they are or how to "treat" them. Like their biological counterparts, computer viruses spread by finding new "bodies" to infect; all they have to do is slip into a computer's address book and harvest a new batch of e-mail addresses. And also like their biological counterparts, they are a product of globalization.

One has only to look at relations between the United States and China to see how globalization has changed the world. President Nixon's 1972 visit to China shattered the bamboo curtain that had effectively separated East from West. His diplomatic overtures led to a renewal of trade with China. Two-way commerce between the U.S. and China totaled $12 billion in 1990 and more than $175 billion by 2003—most of it in the form of Chinese goods headed west. Furniture, toys, pots and pans, consumer electronics, clothes, and artificial flowers are among the myriad items in U.S. stores that are stamped "Made in China."

This flow of goods is made possible by human contact—at West Coast docks, at hotels, at airports, and at trade conferences. Each contact brings the risk of transmitting a new disease such as SARS or the latest variation of influenza, which, like many of the new diseases, often jumps species from animals to humans.

Global travel leading to the spread of disease is not a new phenomenon. To cite just one example from the past, the native populations of the Americas were decimated by the arrival of European diseases to which they had no immunity. But the scope of the problem has been magnified by air travel. There are now more than 500 million international border

In 2004, the World Health Organization reported outbreaks of these infectious diseases. The diagram above shows the main regions where they occurred.

crossings each year by air. And jets make it easy for business travelers and tourists to get to places they'd never have visited in the past. Rain forest tourism, for example, may expose travelers to new, unknown organisms spawned in the fertile DNA-mixing factories of those hot, moist environments. AIDS is believed to have originated in Africa's rain forests; in another era, it might have remained there.

Similarly, say some experts, West Nile virus once caused a relatively mild disease and was found only in Africa. Then it jumped to the Middle East and Europe, and by the time it got to the U.S. it had become a powerful pathogen that has debilitated many of its victims and killed others.

The speed and scope of travel in the 21st century mean the U.S. (or any other country, for that matter) may get an outbreak under control one day, only to have new cases deplane the next day. And travel is just one factor. Three others are at work, according to D.A. Henderson, M.D., M.P.H., who headed the World Health Organization's successful drive to wipe out smallpox. They are urbanization, the internationalization of the world's food supply, and the increasing number of hospitals in developing countries.

"In 1955, there were two cities with populations greater than five million—New York and London," he says. "The number of urban areas with more than 15 million people now numbers more than 15. Many of these are in developing countries, where nutrition is poor, sanitation is poor, and there are minimal health resources. So if an organism mutates to become more lethal and more contagious, it can really get a foothold and spread rapidly.

"The quantities of food coming from abroad are prodigious," he continues. "And that food may carry the same pathogens as if you had bought it in, say, Indonesia. I tell people you don't have to travel to get these diseases. You can get them right at home."

It's ironic that the growth in the number of Third World hospitals has worsened the problem of global disease transmission. Henderson says that's because hospitals in resource-poor nations lack essential equipment, so syringes and needles are routinely reused. "It's reasonable to think HIV was spread by this practice," he adds.

Many Americans still think of AIDS as a disease that afflicts gay men. They may also think of it as a disease that has put the U.S. health system under financial stress. But the biggest threat of AIDS is that it is destabilizing large sectors of the developing world, particularly sub-Saharan Africa, where the disease is spread in part through heterosexual prostitution and regional trade. A Rand Corporation examination of globalization and disease cites studies that tracked the spread of AIDS along African trucking routes. A sampling of 68 truck drivers and their assistants found that more than a third were HIV-positive. The disease then spreads from trucking routes to nearby settlements. As infected individuals sicken and die, leaving families destitute and children orphaned, the economic and social fabric of the society begins to disintegrate.

"Control or even eradication of infectious diseases is now a global issue, not a local issue," says Ford von Reyn, chief of infectious disease at DHMC. "Microbes have the ability to develop resistance. New ones develop as the old ones are eradicated."

Facts like these may seem of little import to well-off Westerners. "The average American doesn't give a damn about AIDS in Africa," says James Strickler, M.D., former dean of DMS and longtime cochair of the board of the International Rescue Committee, a global relief agency. The average American's attitude, Strickler continues, is "'What's AIDS in Africa to me.' Well, it isn't just that AIDS