DENVER - The number of Americans hit by West Nile virus is down nearly 75 percent from this time last year, but health officials warned Friday that people should not drop their guard because the peak season is still ahead.

"We don't want people to get complacent," said John Pape, an epidemiologist with the Colorado Department of Public Health and Environment.

Nationwide, 109 human cases have been reported this year, compared with 406 at this time last year, said Dr. Theresa Smith, a medical epidemiologist with the national Centers for Disease Control and Prevention in Fort Collins.

Smith cautioned that the West Nile season usually peaks in August, "so we do have some wait-and-see."

The mosquito-borne virus, which can cause serious neurological diseases, first appeared in the United States in New York City in 1999 and spread west. Since then, the CDC has counted 16,814 human cases and 659 deaths nationwide.

West Nile reached Colorado in 2002, with 14 human cases but no deaths. It peaked in Colorado in 2003, killing 63 people and sickening 2,947, most in the nation. This year, 13 human cases have been reported in Colorado, compared with 103 at this time in 2004.

"I'm certainly at this point feeling pretty comfortable were not going to have an '03 year," Pape said.

The reasons for the slow start this year aren't clear.

"I don't know that I have that answer, or that anybody has that answer," Pape said. "We're still trying to feel our way through this."

He said many local health departments have learned effective ways to fight the disease by starting mosquito control early and eliminating standing water where the insects can breed.

Mesa County in western Colorado was the "epicenter" for the virus in the state last year, but county officials contained it quickly, Pape said. "They just shut down their outbreak. I mean, just shut it off," he said.

The county health department declared a public health emergency, spraying 211,000 acres for mosquitoes, launching a publicity campaign and distributing insect repellent.

"What we saw was a dramatic decrease in the number of cases of human West Nile" after spraying began, said Dr. Michael Aduddell, director of the county health department. He said 22 new cases were reported the week before spraying started and 10 were reported the week after.

Countywide, 127 people contracted West Nile last year and four died.

Pape said weather patterns, moisture, temperature and the population of mosquitoes and birds - which can transmit the disease to mosquitoes - could all play a role in how severe this season's outbreak is.

"When all the factors come together in the right order, you could have some hot spots," Pape said. The specifics of how the factors interact are difficult to pin down, he said.

He said the experience of Eastern states shows West Nile is unlikely to disappear anytime soon.

"In none of these states has the virus gone away. What we expect, as with most mosquito-borne diseases, is this virus will stick around."