Denver ? A fifth person has died from the West Nile virus in Colorado, the state hardest hit by the disease this year.

The 67-year-old Boulder woman died Tuesday, six days after the onset of West Nile symptoms including encephalitis, Boulder County Public Health officials said Sunday. Lab results returned late Friday confirmed she had the virus, said Heath Harmon, a coordinator with the county health department.

Colorado, which had a dozen human cases last year, leads the nation with 166 so far this year, according to state officials.

Why Colorado has seen such high numbers of the mosquito-borne disease is unclear to experts. Some have blamed the outbreak on a wet June. Others have said the weather doesn’t explain the numbers.

Colorado differs from other states because it reports mild cases of the virus that some do not report, state epidemiologist John Pape has said.