A woman in Long Island recently fell ill with a mosquito-borne virus rarely seen in the United States. She had not traveled abroad. The virus, called chikungunya, causes fever and extreme joint pain. It is usually found in regions of East Africa, Latin America and Southeast Asia, not in suburban New York. Its arrival in New York is troubling—and part of a larger trend. Diseases once confined to tropical climates are now in the U.S. and more people are falling sick as a result. For years,