A strain of Dengue fever, absent from parts of the Pacific for decades has been found in the FSM.
Dengue: An acute infectious, eruptive, febrile disease caused by four antigenically related but distinct serotypes of the DENGUE VIRUS. It is transmitted by the bite of infected Aedes mosquitoes, especially A. aegypti. Classical dengue (dengue fever) is self-limiting and characterized by fever, myalgia, headache, and rash. DENGUE HEMORRHAGIC FEVER is a more virulent form of dengue virus infection and a separate clinical entity. (From Dorland, 28th ed)
As defined above, dengue is usually spread by the bite of infected mosquitoes. The finding coincides with FSM's rainy season, as well as the holiday season, when many locals travel to and from the islands. This makes the possibility of a spread, likely.