Low water levels, like at this reservoir near Gustine, Calif., bring birds and mosquitoes together and help transmit West Nile virus to humans. Rich Pedroncelli/AP hide caption

itoggle caption Rich Pedroncelli/AP

Small but costly: Dozens of mosquito species carry West Nile virus in the U.S. Darren McCollester/Getty Images hide caption

itoggle caption Darren McCollester/Getty Images

The mosquitoes that feed on people are attracted to over 300 gases and other compounds emitted by human skin. CDC Public Health Image Library hide caption

itoggle caption CDC Public Health Image Library

People living in affluent neighborhoods in north central Dallas were most likely to get infected in 2012. Those neighborhoods were also hit hardest in the 2006 outbreak. JAMA hide caption

itoggle caption JAMA

Mosquitoes are sorted at the Dallas County mosquito lab in Dallas, Texas on Aug. 16, 2012. Dallas County has seen the highest number of cases of West Nile virus of any county in Texas: 379 this year, as of Oct. 25. LM Otero/ASSOCIATED PRESS hide caption

itoggle caption LM Otero/ASSOCIATED PRESS

Technicians with the Contra Costa County Mosquito and Vector Control District spray insecticide in Brentwood, Calif., last month. Workers fogged areas of the county that had an increase in the numbers of mosquitoes carrying the West Nile virus. Justin Sullivan/Getty Images hide caption

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A Beechcraft airplane sprays insecticide over Dallas early Monday morning to curb the spread of West Nile virus. LM Otero/AP hide caption

itoggle caption LM Otero/AP

Christopher Doll releases fish into the water of a neglected pool to kill mosquitoes that might carry West Nile Virus in Concord, Calif., in 2009. Justin Sullivan/Getty Images hide caption

itoggle caption Justin Sullivan/Getty Images