Another person with similar symptoms is under observation.
About one in 150 will develop serious and potentially life-threatening conditions that include encephalitis, which is inflammation of the brain, or meningitis.
Symptoms associated with West Nile virus typically appear 3 to 14 days after infection. According to World Health Organisation, there is one reported case of transplacental (mother-to-child) WNV transmission.
"Mosquito-borne diseases overall are increasing", CDC spokeswoman Kate Fowley told weather.com.
West Nile virus is spread by the bite of an infected mosquito.
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"Mosquitoes are weak flyers and won't travel far from where they are born - if there are mosquitoes in your backyard or neighborhood, they are likely laying eggs there as well", said Matt Helwig, a biolgist with the DEP mosquito program. A very small proportion of human infections have occurred through organ transplant, blood transfusions and breast milk.
People infected with West Nile virus may show no or very mild symptoms. Fairfax County health officials said they had five reports of people contracting West Nile virus a year ago but none in 2016.
The state is checking the case of an Allegheny County resident, sending samples to the Centers for Disease Control and Prevention.
"Public Health thanks the community for being diligent in removing standing water and eliminating the places where mosquitoes can breed", Callanan said.
Since mosquitoes are attracted to standing water, residents are also advised to clean or empty out items that may hold water such as old tires, buckets, planters, toys, birdbaths, flowerpots, tarps, trash containers, downspout extensions and other containers to stop mosquitoes from breeding on your property.