The trap was part of the county's routine West Nile Virus Vector Surveillance testing.
According to the Department of Public Health, the St. Louis area is home to more than 50 different species of mosquito.
The Health Department said the public can assist in identifying areas with water which could become potential breeding sites for the mosquitos, which lay their eggs in water.
Cases generally tend to spike during the summer and fall months, between June and September, when activity is the most common.
'Firenado' sweeps across factory field in England
Firefighters were joined by colleagues from other counties as they tried to prevent the fire spreading. Thick black smoke from thousands of plastic pallets on fire could be seen across the East Midlands .
The CCHD says that the infected person is over the age of 65 and was exhibiting WNV symptoms, which led to being tested, but is not able to release any other patient-specific information due to privacy laws.
West Nile fever generally resolves itself without treatment, but unsafe brain infections such as encephalitis or meningitis can develop in 1 out of 150 people, according to the press release. Less than one per cent develop more serious central nervous system issues, she said.
The Centers for Disease Control and Prevention said the symptoms of severe cases include high fever, headache, neck stiffness, stupor and disorientation. MDH says there were 30 West Nile cases last year and 83 the year before.
This year, West Nile has not yet been found in a Wisconsin resident. Earlier this week, the Montana Department of Livestock confirmed the first reported cases of equine West Nile Virus in Montana of the year.
Wear long sleeves and trousers or wear repellent to help protect yourself from getting bitten.