Officials say Monday that the person who tested positive is a Box Elder County resident between the ages of 18 and 39.
West Nile symptoms include high fever, a severe headache and stiff neck, disorientation and confusion.
While the mosquito-borne illness is rare in humans, Utah has counted 327 cases and eight deaths from the virus since 2003.
Health officials encourage people to protect themselves from mosquito bites and remove puddles or standing water where the insects might breed.
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