On Wednesday, New Mexico (search) reported its first human case. At a news conference Thursday, Dr. Jonathan Weisbuch, director of the Maricopa County Department of Public Health, said an adult from the county developed symptoms May 8 and was admitted to a hospital.
Weisbuch said that person has now fully recovered.
In all, eight residents from Maricopa County, which includes Phoenix, contracted the virus last year, but only one was infected within the county, Weisbuch said. That case -- a person who became ill in September and has now recovered -- was confirmed by the CDC as West Nile virus earlier this month.
Arizona's mosquito season usually runs from August through September after the monsoon rains, and "this is much earlier than we expected," Weisbuch said. "It's going to be a longer season."
"We could see several hundred cases," he said.
Dr. Amy Bode, a medical epidemiologist for the Division of Vector-Borne Infectious Diseases at the CDC laboratory in Fort Collins, Colo., warned against predictions.
"The second year is not necessarily [the worst] but it can be," she said. "The states should try to be ready and take precautions."
West Nile, which first hit the United States in 1999 in New York, has killed more than 560 people in the United States in the past five years.
Last year was the first for West Nile to appear in areas west of the Continental Divide.
Colorado led the nation last year with 2,947 of 9,862 human cases reported to the federal Centers for Disease Control and Prevention. The disease resulted in 61 deaths in that state.
Health officials said the virus typically causes flu-like symptoms such as fever, nausea, headache and muscle aches. But in some cases, it progresses to life-threatening encephalitis or meningitis. People age 50 and over are generally at a higher risk for severe symptoms.