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In this photo provided by the Northwest Mosquito Abatement District, lab assistant Carter Sharp sorts mosquitoes Thursday, July 26, 2012, in Wheeling, Ill., that were collected from Chicago's northwest suburbs for West Nile virus testing. Illinois already has seen at least two human West Nile virus infections this year, more than a month earlier than usual. The rate of infected mosquitoes in the state is the highest in a decade and higher than in 2002 when 67 people died. Health officials say the hot, dry weather is a factor. (AP Photo/Courtesy the Northwest Mosquito Abatement District, Michael Szyska)

West Nile virus outbreak

The United States is experiencing its worst seasonal outbreak of the mosquito-borne West Nile virus. A total of 3,545 cases, including 147 deaths, were reported in the United States in 2012, the highest human toll by that point in the calendar since the mosquito-borne disease was first detected in the country in 1999.