Infected mosquitoes were collected from Selden, New York, the health department said.
The sample was the second to test positive for West Nile Virus this year in Suffolk County. The first was found in South Huntington earlier in June.
Health officials in New York City announced just last week that the virus had been found in a sample of mosquitoes from Queens and Staten Island.
Officials say that no cases of the virus have been reported in humans in Suffolk County or NYC this year.
At least five people in four of NYC's boroughs contracted the virus last summer, according to the NYC Department of Health. Three of the patients had to be hospitalized.
West Nile Virus was first detected in birds and mosquito samples in Suffolk County in 1999 and again each year thereafter, according to the county's department of health.
Most people infected with the mosquito-borne virus either show no symptoms or suffer a flu-like illness. A small number develop a serious illness that can lead to neurological damage.
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Protective measures against mosquitoes include eliminating standing water, using repellent, wearing long-sleeve shirts and long pants, and repairing doors and screens for tight fits.
