Officials say a Chesterfield adult has recovered from the infection - the first since August 2012.
Since West Nile was first identified in New Hampshire in 2000, nine human cases of the virus have been reported.
Health officials caution that it is not clear where in the state the Chesterfield resident was infected. They say residents statewide should take precautions, as the disease so far this year has been detected in eight batches of mosquitoes. Those batches came from Kensington, Nashua, New Castle, Pelham, Sandown and Stratham.
West Nile in most adults produces mild flu-like symptoms or none at all.
Those over 50 are at higher risk of illness.