The virus has been a part of public health discussions since it first arrived in the United States in New York in 2001, said Evanston's assistant director of health and human services, Carl Caneva. In a typical year, he said, NSMAD traps mosquitoes that test positive for the virus around the end of June, birds carrying the virus are usually found later on in the season, while human infections are reported usually toward the end of summer.