CANTON, Ill. -- Within weeks, if not days, nature will quietly snuff out a killer in Illinois.
The first hard frost will put West Nile in the deep freeze, halting a statewide scourge that has infected 691 people and killed 43 -- by far the deadliest outbreak since the virus was first detected in the United States in 1999. The cold will kill the mosquitoes that carry the virus.
But health officials know the relief could be as fleeting as frost on an autumn morning. So they will spend the winter studying everything -- including the outbreak's geography, its timing and its victims -- that might help them ward off the virus next year.
As of Monday, there were 3,231 reported human cases of West Nile virus in the United States this year and 176 deaths, according to the Centers for Disease Control and Prevention.
Experts are uncertain why the problem is so bad in Illinois, but their theories include bird migration patterns, a heavy concentration of mosquito-infested cemeteries, and a landscape that includes lots of forests and marshes.
Over the winter, experts will analyze where in Illinois the disease hit hardest. The Chicago area has logged more than two-thirds of the cases, while downstate Canton, a city of just 14,000, had two deaths.
They will also look at the timing of the outbreak, which registered its first human case in early August, peaked in early September and began tapering off in early October.
And they will research West Nile's victims. Horses proved most vulnerable, with nearly 1,000 confirmed cases and a 42 percent mortality rate. For reasons that are still a mystery, other livestock and pets were virtually unscathed.
In the meantime, health officials are hoping history repeats itself. In 1975, a mosquito-borne outbreak of St. Louis encephalitis in Illinois rivaled this year's West Nile crisis, with 578 confirmed cases. The next year, that fell to 19, then to zero by 1977.
One theory is that birds -- a primary carrier of the virus -- develop an immunity and no longer pass the disease to mosquitoes.
Even if nature fails, Tom Schafer, spokesman for the Illinois Department of Public Health, said he expects West Nile numbers to drop next year. The key to prevention is public awareness, which increased as the toll climbed, he said.
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