As it turns out, the historic drought plaguing California could be a major contributor to the increase.

Although it might seem that mosquitoes, which depend on water to breed, would suffer when there's less water on the ground, the opposite is true under the right conditions. Droughts, along with warm weather, can produce the conditions necessary for an abundance of the insects.

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In the Southern California region at the center of the state's West Nile outbreak this year, the drought conditions are just right for the flourishing of a particular species of mosquito: the southern house mosquito, or ulex quinquefasciatus, which, according to the Centers for Disease Control and Prevention's Roger Nasci, is a "very good vector for the disease."

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Nasci, chief of the CDC's arboviral diseases branch, said in an interview that drought conditions this summer have helped "cook down" the pools of water favored by the species, so that those habitats "get nice and stagnant and stinky. This mosquito likes it a lot." The rainwater collection vessels that are popular with conservation-minded residents of drought-ridden areas, for instance, can be perfect for mosquito reproduction unless steps are taken to discourage it.

Because mosquitoes usually pick up West Nile virus from infected birds, the shortage of water also helps increase the share of insects carrying the disease. "When we have less water, birds and mosquitoes are seeking out the same water sources, and therefore are more likely to come in to closer proximity to one another, thus amplifying the virus,” California's department of public health chief Vicki Kramer told KQED.

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In other words, the drought brings together a common host of the virus with the insect that transmits it to other animals, including humans.

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Kramer added that stagnated backyard pools and bird baths are also perfect breeding grounds for mosquitoes. In some places, such as Orange County, local officials attempting to spray those pools with pesticides have run into an added obstacle: Foreclosed properties with abandoned pools, which create wonderful breeding grounds for mosquitoes that often go under the radar of those sweeping the area to try and prevent them.

It's been a problem in past years, as KQED noted, and continues to boost conditions for the spread of West Nile this year. There have been 116 cases of West Nile in Orange County alone this year.

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A concentrated outbreak like that is "pretty characteristic of West Nile virus," the CDC's Nasci said. However, the factors at play in Southern California are hardly the only ones favorable to the spread of West Nile. The virus is found "east to west, north to south, in pretty much every ecosystem in the country," Nasci noted, with different conditions and the habits of different mosquito and bird species at play in other outbreaks.

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And not all droughts are created equal as far as mosquito reproduction is concerned. "If the drought is too extreme, you don't get any mosquitoes," Nasci said. Temperature is also a factor: Mosquitoes like it warm.