Infected travelers on international flights can spread disease, but Norman Gratz, a medical entomologist with the World Health Organization, worries more about the nonhuman passengers. At least 89 people have contracted malaria from African mosquitoes inadvertently set free around international airports over the last 30 years, and Gratz believes many more cases may have gone unrecognized. "Physicians dealing with a patient who has not recently been out of the country will hardly ever think of malaria as a diagnosis," he says. He warns that hard-to-diagnose viral diseases such as dengue fever could become established in airport insect populations and then spread. West Nile virus may have reached New York City this way.
If current warming trends continue, airports in Europe and the United States will be increasingly friendly to tropical diseases. And airplane stowaways would likely be especially effective at reintroducing diseases to climatically favorable regions where they were previously eliminated. Gratz proposes giving aircraft a "disinsection" once a month and spraying cabins immediately before passengers board. What about the risks of exposing passengers to insecticides? "Diseases such as malaria and West Nile virus are much more dangerous than the spray," Gratz says.