Moving people, animals and insects, air travel creates a kind of globalization of illness. "People are going across the world so rapidly that they're bringing the disease with them," explains Patrick Moore, a research scientist who has studied epidemics. Moore believes other factors -- like weather -- can in effect wake-up viruses that may have existed in the United States for years without infecting humans.
He cites "changes, such as changes in irrigation, in farming and water use. Changes in the global environment, through global warming. Changes in the severity of winters."
It could take years to determine how West Nile virus arrived. Scientists faced a similar puzzle after the 1993 outbreak of the deadly Hanta virus in New Mexico. They eventually traced it back to a wet spring and summer that caused an explosion in the mouse population.
"The mouse is the carrier for this virus, so you had a lot more opportunities for person-mouse contact," Moore explains.
Dr. Ian Lipkin, who discovered the West Nile virus in the U.S., warns the country is no longer immune to the spread of all sorts of infections. "As we enter this new millenium," he warns, "there are going to be increasing frequencies where we see new agents brought to the United States."
Small outbreaks of exotic diseases are already popping up: Dengue Fever in Texas, Malaria in California, and more infections of Hanta virus in and around New Mexico.
And population trends are scaring some doctors. The U.N. estimates that by the year 2025, almost two-thirds of the world's population will live in cities. That means if there's an outbreak of contagious disease, it could spread much faster than it used to.