Parts of northeast Texas were sprayed with insecticide early Friday in an aerial assault on mosquitoes as more cases of West Nile Virus were reported as far north as Illinois.

Planes began spraying an area of Dallas north of Interstate 30 and east of the Dallas North Tollway from 10pm local time (11 p.m. ET) Thursday night until the early hours of Friday, NBCDFW.com reported. More areas were expected to be sprayed later Friday.

Ten people in Dallas County have died after contracting the virus, and hundreds more have been infected.

Health officials confirmed a second case the mosquito-borne virus in DuPage County, Ill., according to a report in the Chicago Tribune on Friday.

According to the Center for Disease Control, there have been 693 confirmed cases across the United States, and 26 deaths.

It is the country's worst outbreak of the disease since it was first recorded in 1999, The New York Times reported.

The hot, dry weather across the country's midsection has created ideal conditions for some species of mosquito. The heat speeds up their life cycle, which accelerates the virus replication process. During a drought, standing water can quickly turn stagnant when it is not flushed away by rain or runoff.

More on this story from NBC News' affiliate NBCDFW in Dallas-Fort Worth

"This year is totally different from the experience Texas has had in the past," state Health Commissioner Dr. David Lakey told NBCDFW.com. "If it's nuisance mosquitoes, we ask the city or county to pay part of that. But in the midst of this disease outbreak, it's easier for us to go ahead and do it."

A national spraying company, Clarke Mosquito Control, was set to deploy two to five Beechcraft King Air twin-engine planes late Thursday night for three hours of spraying.

One county-wide application costs about $1 million. A second application is possible if the first attempt does not kill enough mosquitoes.

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