Health officials said Tuesday that a woman died from West Nile virus, the state's sixth death from the mosquito-borne disease this year.
The 80-year-old Loveland resident died at a hospital there, said epidemiologist John Pape. He would not reveal the woman's name or when she died.
Colorado has been the state hardest hit by West Nile, with at least 215 human cases.
As people buy gadgets, bug spray and citronella candles to keep the mosquitoes away, cities are also trying to stop mosquitoes from hatching.
In parts of Boulder County, home to one of the victims, trucks slowly drive down streets misting the air with mosquito pesticide.
Workers in Colorado Springs are putting larvicide in 2,000 stormwater basins where mosquitoes have been breeding. The larvicide stunts the growth of mosquitoes. It also prevents larvae from developing into biting adults.
Why Colorado has seen such high numbers of the mosquito-borne disease this year is unclear to experts. Some have blamed the outbreak on a wet June and very hot July, which they say provided the perfect summer for mosquitoes. Others have said the weather doesn't entirely explain the numbers.
"I can't predict what will happen in Colorado, nor can I completely explain why it is happening," said Dr. Julie Gerberding, director of the CDC.
"That certainly makes us very nervous," said Craig Levy of the Arizona Department of Health Services. "If it can increase that dramatically in Colorado, it has the potential to do so in Arizona."
Experts expect the mosquito-borne disease — which killed 284 people nationwide last year — to reach every state in the coming months.
Colorado differs from other states because it reports mild cases of the virus that some do not report, Pape has said.
The CDC had confirmed only 72 Colorado cases and only one of its deaths as of Friday, according to the organization's Web site. Nationwide, the agency confirmed 182 West Nile cases and five deaths.
The virus is spreading fast nationwide and will likely break last year's record 4,156 cases, Gerberding said.
"The numbers are starting to change very, very quickly," Gerberding said last week. "That is very concerning."
"It indicates we are starting the epidemic with more cases than last year," the CDC director said. She warned of "a great number of infected people."
New cases are likely until the first frost of the year kills the mosquitoes. The good news is that the numbers tend to decrease after a state's second year dealing with the disease.
"Ever since West Nile virus arrived, we've always been forecasting that this year would be the worst year for infections in humans," Heath Harmon, communicable disease control coordinator for Boulder County Public Health. "For the rest of this summer and into next year, we'll try to weather the storm."