The spread of the West Nile virus in the U.S. has put health authorities on alert.

The U.S. Centers for Disease Control and Prevention said West Nile has killed 219 people this year, including 36 who died last week.

The figure is the largest since 2003. The death toll is expected to increase in the future, as patients can die several weeks after they are infected with the virus.

The U.S. Centers for Disease Control and Prevention has received four-thousand-725 reports of West Nile virus infection this year, the largest in nine years.