Prof. Thomas Scott, a UC Davis entomologist, says that distribution of the mosquito "tends to be patchy." Even in places where it's very common, like Thailand, he said, "distribution will fluctuate, even through the course of a season."
The Aedes aegypti mosquito was introduced to the Americas from Africa, likely on slave ships, perhaps as early as the 16th century, Georgetown historian Prof. John McNeill told CNN. Along with it came yellow fever, dengue, chikungunya and now Zika virus.
While the spread of the virus has been very rapid across much of South and Central America, Scott says it is less likely to spread as swiftly in the U.S. primarily because "our lifestyle is different than places where [many] people are infected."
In the U.S. window screens or air conditioning are common, so mosquitoes are less likely to be in people's homes. In Central and South America, people's homes are more open, the "mosquitoes move frequently [in homes] and bite frequently," Scott noted.
Case in point: the Aedes aegypti mosquito has been present across southern Arizona "for a long time," Scott says, but the risk of dengue fever in Arizona is low. Meanwhile, just across the border in Mexico, the risk of dengue to people is much higher.
Zika spreads when a mosquito bites a person who is infected. Then the mosquito becomes infected itself. But Scott says it cannot spread Zika to another person right away. The virus must replicate within the mosquito first. Then the virus must travel to the insect's salivary glands and replicate there. Only then has the mosquito moved from being infected to being infective, Scott said, and that takes awhile.
"That period of time is about the average life span of the mosquito, 10-14 days, for them to become infective," Scott says. In other words, it's possible the mosquito could die before it's able to infect another person.
Zika can also be sexually transmitted and a pregnant women can transmit the virus to her fetus during pregnancy, according to the CDC.
The California Department of Public Health lists the cities where Aedes aegypti has been found. It did not get back to us by deadline to tell us when the mosquito was found in these places.
Fresno: Clovis, Fowler, Fresno, Kerman, Mendota, Sanger
Imperial: Andrade, Brawley, Calexico, El Centro, Heber, Holtville, Imperial, Seeley
Los Angeles: Commerce, East Los Angeles, Florence, La Mirada,Los Angeles, Maywood, Montebello, Pico Rivera, South Gate
Madera: Madera, Madera Ranchos, Parkwood
Orange Anaheim, Costa Mesa, Garden Grove, Lake Forest, Mission Viejo, Orange, Santa Ana
Riverside Coachella, Riverside, San Jacinto
San Bernardino: Colton, Montclair
San Diego: Bonita, Chula Vista, El Cajon, Escondido, Imperial Beach, Oceanside, San Diego, Spring Valley, Tecate, Vista
San Mateo: Atherton, Menlo Park