DALLAS -- West Nile deaths were mounting quickly this summer as mosquitoes spread the virus across the country. The situation was especially dire in Texas, where some authorities resorted to aerial spraying for the first time in decades to curb what became one of the worst such outbreaks in U.S. history.Nationally, more than 240 people died from the mosquito-borne illness -- about a third of them in Texas.Now with the mosquito population decimated by the cooler weather, health experts have turned their attention to learning lessons from the latest round of deadly cases. Federal health authorities are collecting data and examining potential factors, while Dallas County -- the epicenter of the outbreak -- has begun year-round mosquito surveillance and testing.What remains unclear is whether experts will be able to shed light on what caused the outbreak, why parts of Texas were so severely affected and whether they can forecast the next major surge in the illness."I don't think that we're ever going to totally be able to sort it out," said Dr. Lyle Petersen of the Centers for Disease Control and Prevention. "For one reason, the ecology in the United States is extremely varied, so a factor that may affect an outbreak in Colorado may be different than a factor that causes an outbreak in Louisiana. The conditions in an urban area may be different than a suburban area."West Nile virus is believed to have first appeared in the U.S. in 1999 in the New York City area and then gradually spread across the country. Mosquitoes get the virus from feeding on infected birds and then spread it to people they bite. Most people infected show no symptoms, but the most severe form of the disease, called neuroinvasive, can cause a coma, convulsions, muscle weakness, paralysis and death.The Texas Department of State Health Services reported more than 835 neuroinvasive cases this year and 86 deaths, led by Dallas County's 18 fatalities. In Tarrant County, eleven deaths were reported from the virus, according to Tarrant County Public Health. Tarrant County health officials documented 275 cases of West Nile virus, 175 cases of West Nile fever and 100 cases of West Nile virus neuroinvasive disease.The national death toll approached historic numbers from 2002, when 284 people died from the disease.Petersen said the CDC is trying to determine what caused the latest outbreak by looking at factors such as heat, rainfall and the number of migrating birds that transmit the virus to mosquitoes. The agency is also researching the genetics of the virus to see if it mutated, but that doesn't seem to have happened.Petersen added that while a warm spring with ample rainfall in North Texas was likely a factor, it's not known exactly what caused so many cases in the area."Probably, there was just the right combination of warmer weather and enough rainfall and probably a good dose of bad luck as well," Petersen said. "These outbreaks are subject to a fair amount of random variation."The situation became so severe in North Texas that the state paid for aerial spraying in areas of Dallas County and nearby Denton County. Dallas County had not conducted such an operation since 1966 when encephalitis was blamed for more than a dozen deaths.The county began spraying from trucks with the first positive mosquito test in June and then added aerial spraying in August when it became apparent there were more areas with infected mosquitoes than could be covered with spray trucks, Dallas County Judge Clay Jenkins said."If you look at this outbreak, it came upon us very suddenly," Jenkins said. "We might have the same situation in the future where by the time people know there is a risk out there, dozens of people have already been infected."Public health officials say they will begin public information campaigns earlier next year, advising residents to apply insect repellent, wear long sleeves and long pants when outside, stay indoors from dusk to dawn and drain standing water on their property.