The return of West Nile virus: A guide

West Nile virus is back this summer, with a vengeance. It has hit especially hard in Dallas, but the mosquito-born illness has been reported elsewhere in Texas, too, and in dozens of states. Why the big outbreak now? Here, a brief guide:

How bad is the outbreak?

So far, 693 cases have been reported in 32 states, with 28 deaths nationwide, according to the Centers for Disease Control and Prevention. The most intense outbreak has been in Texas. In Dallas, the epicenter, 10 people have died and at least 230 others have fallen sick with the West Nile fever, suffering headaches, joint pains, vomiting or diarrhea, and rash. That's twice as many cases — and two more deaths — than had been recorded as of last week.

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Why is it coming back so strong?

A mild winter followed by ample spring rains allowed blood-sucking mosquitoes that can carry the virus to start breeding early. Then hot, dry weather created ideal conditions in the middle of the country for mosquitoes to thrive. Heat speeds up the bugs' life cycle, so the virus replicates faster. During a drought, there's little rainfall to flush out standing water, so it stagnates, creating perfect incubators for mosquitoes.

Can anything be done do fight the problem?

There's no vaccine or proven therapy. But Dallas officials, who have declared a state of emergency, are attacking the problem with their first aerial pesticide spraying in four decades. The idea isn't universally popular — 1,400 people have signed a petition in protest, questioning whether the spray is safe and warning that it could kill beneficial insects. Dallas Mayor Mike Rawlings says the pesticide is not dangerous to humans, so it's the lesser of two evils. "I cannot have any more deaths on my conscience because we did not take action," he says.

Sources: Associated Press, CBS News, Daily Beast, USA Today