"The hotter it is, the mosquitoes tend to be more infectious, and it also affects how long a mosquito may live," Petersen said.
Floods wash out mosquito breeding sites. But the right amount of rain can produce ideal breeding conditions.
Robert Haley, director of the epidemiology division at the University of Texas Southwestern Medical Center at Dallas, said he suspects that particular local weather conditions made Dallas ground zero for West Nile virus this year.
A mild winter allowed more female mosquitoes to survive. West Nile infection among birds was relatively mild last year, meaning more birds would be susceptible this year. Haley, who lives in North Dallas, said he suspected that something was amiss when he saw two dead bluejays in his yard in July. Bluejays and crows are among those species that tend to die from the virus, he said.
Dallas also had a hot, dry summer with rain every three to four weeks that replenished the stagnant pools in which mosquitoes breed, he said.
West Nile disease can vary in severity. The onset of symptoms can take from a few days to two weeks. People 50 or older have the highest risk of severe illness.
About 80 percent of people who are infected will not develop any illness. About 20 percent will develop West Nile fever. Symptoms include fever, headache, tiredness and body aches. Occasionally, there will be a skin rash and swollen lymph glands.
The most severe type of infection causes inflammation of the brain or of the tissues surrounding the brain and spinal cord. In those cases, symptoms include headache, fever, stiff neck, muscle weakness and paralysis.
Of the West Nile disease cases reported to the CDC this year, 1,069, or 54 percent, were considered severe.
Health officials say residents should use insect repellent when outdoors, especially at dusk and dawn, when mosquitoes are most active. Residents should also eliminate mosquito-breeding areas by emptying birdbaths, flowerpots, buckets and barrels where rainwater collects.