On February 11, 2001, an enormous cloud of dust whipped out of the Sahara Desert and moved north across the Atlantic, reaching the U.K. two days later. A few days afterward, counties across the island began reporting simultaneous outbreaks of foot-and-mouth disease, a viral sickness of livestock (sometimes confused with mad cow disease). For Eugene Shinn, a geologist at the U.S. Geological Survey in St. Petersburg, Fla., that coincidence suggested an obvious link.

The idea that large-scale disease outbreaks could be caused by dust clouds from other continents has been floating around for years. But it seemed far-fetched. In the U.S. government, "no one wanted to listen to me," Shinn remembers about his proposal that something as amorphous and uncontrollable as a dust cloud could bring the disease to America.

But the theory is now gaining acceptance as scientists find that it may explain many previously mysterious disease outbreaks. Although the world's dry areas have always shed dust into the atmosphere--wind blows more than a billion tons of dust around the planet every year--the globe's dust girdle has become larger in recent years. Some of the changes are part of nature's cycles, such as the 30-year drought in northern Africa. Others, including the draining of the Aral Sea in Central Asia and the overdependence on Lake Chad in Africa, are the result of shortsighted resource management. Poor farming practices also hasten desertification, creating dust beds polluted with pesticides and laced with diseases from human and animal waste.

For Shinn and his co-workers, it was a strange disease outbreak in the Caribbean in the early 1980s that first brought to mind the connection between dust and disease. A soil fungus began to attack and kill seafan coral. The researchers doubted that local human activity was the culprit, because the disease was found even in uninhabited places and islands devoid of soil. In addition, Garriet W. Smith of the University of South Carolina demonstrated that because the soil fungus could not multiply in seawater, it required a constant fresh supply to continue spreading.

Smith analyzed the African dust blowing across the Caribbean and was able to isolate and cultivate the soil fungus Aspergillus sydo-wii, with which he infected healthy seafans. USGS investigators then showed how the Aspergillus fungus and other organisms could survive the long trip from Africa protected by dense clouds of dust.

Researchers are now finding evidence that supports the link between sickness and dust. Ginger Garrison of the USGS believes that there is a direct link between bacteria-caused coral diseases such as white plague and black-band disease and African dust storm activity. In addition, outbreaks of foot-and-mouth disease in South Korea last year followed large dust storms blowing in from Mongolia and China.

Other organizations are now joining the USGS in tracking dust. NASA has satellites that are carefully monitoring dust storms, which can cover an area as large as Spain. The National Oceanic and Atmospheric Administration has just opened a station in California to track Asian dust as it passes over the U.S. (Although the SARS virus could theoretically cross oceans in a dust storm, the epidemiology so far indicates that person-to-person contact is the only way SARS has spread.)

The findings on international dust storms have also attracted the attention of those who are concerned about bioterrorism. "Anthrax will certainly make the trip" in dust from Africa to the U.S., remarks Shinn, who recently completed a terrorism risk assessment for the U.S. Dust clouds could be considered, in effect, a very dirty bomb.

Otto Pohl is based in Berlin.