DENVER (CBS4) – The Centers for Disease Control and Prevention is calling 2012 the worst year for West Nile virus since 2004.

About 240 people nationwide have contracted the disease and four have died. Most of the cases are in Texas, Mississippi and Oklahoma. Three cases have been reported in Colorado.

The worst part of the season could still be to come.

“We are early in the season. August and September are usually the peak transmission periods, so having this much activity this early and this widespread is cause for concern,” Dr. Roger Nasci with the CDC said.

The CDC says West Nile season may have arrived early because of the mild winter and early spring across much of the U.S. followed up by high temperatures, and then rain. That all adds up to perfect breeding conditions for mosquitoes.

Some areas in Colorado are spraying to reduce mosquito numbers.