Health experts with the U.S. Centers for Disease Control warn that, despite the cooler temperatures of autumn, cases of West Nile virus remain higher than normal across the United States.

So far, North Dakota has reported 352 cases of the virus, compared to 133 in the fall of 2006, and 506 cases have been reported in Colorado, up from 345 the previous year.

Other states that have felt the sting of the virus include California, Montana, Oklahoma and Wyoming.

West Nile is generally found in mosquitoes and birds. However, humans and livestock can be infected if bitten by a virus-carrying mosquito. Stagnant pools of water, where mosquitoes tend to gather, have played a major role in the increased number of cases in the Western U.S.

Additionally, heavy rainfall and flooding followed by hot, dry weather during the summer has worsened the mosquito threat.