DALLAS (AP) The Dallas County Medical Society says the county should do aerial spraying immediately to help reduce the spread of the West Nile virus.

The nation's top health officials say more serious illnesses from West Nile virus have been reported so far this year than any since 2004. Most have been in Texas, especially around the Dallas area.

Mosquitoes pick up the virus from birds they bite and then spread it to people.

Dallas television station WFAA obtained the Dallas County Medical Society's letter that says doctors are becoming more concerned as this season's outbreak has reached historic levels. The letter was sent to the County Commission, which may discuss the matter at its meeting Tuesday.

Aerial spraying has not been done in Dallas since the 1960s.