HOUSTON "Mosquitoes are flying syringes," declared Frank Cortez-Flores of Loma Linda University (California), and two mosquito-borne diseases have broken past old geographic boundaries to invade the US. The first, West Nile encephalitis, is a newcomer to the western hemisphere and thus has garnered the most headlines. The other, dengue fever, is considered the world's most important vector-borne viral disease affecting people, in terms of both morbidity and mortality.

The West Nile virus, native to Africa and undetected in the US until August 1999, is an example of an infectious agent that seems to be spreading beyond its previous boundaries, causing an 'emerging infectious disease'. Because of mutations and other changes in disease-causing organisms, and increases in international travel, the incidences of once-exotic diseases in new territories are increasing. Some suggest that global warming also contributes to the spread of these diseases.

West Nile virus is no stranger to Europe, having...

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