The Southwest Utah Public Health Department says about a fifth of people infected with the virus develop medical complications.
Symptoms can include sudden fever, fatigue, aches and pains, headache and rash.
Authorities are warning people to protect themselves outdoors.
Officials say the positive test results came from mosquitoes trapped recently in Washington County.
There have been no reports of human infection in Utah this year. The last case was in Kane County two years ago.
Public Health surveillance nurse Lisa Starr says about 1 percent of people develop serious illness from the virus.
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