All it takes is a little bit of water and a warm breeze.

Aedes aegypti, the main mosquito species that transmits Zika virus and several other serious diseases, can breed practically anywhere. These mosquitoes will lay eggs in the stagnant water collected in bird baths or old tires, sure, but also in the few droplets gathered in a discarded candy wrapper, or on the damp surface of a fallen leaf, or in a practically-empty soda can.

This startling resilience is part of what makes Aedes aegypti so efficient a vector and, in turn, so serious a threat to humans. Zika was for decades considered to be a mild virus, mostly just a nuisance, but is now known to cause serious health problems including grave outcomes for fetuses like abnormally small heads, severe brain defects, and even death.

In Brazil, Aedes aegypti is at the center of a Zika outbreak that global-health officials estimate will cause more than 2,500 cases of microcephaly. (As of March 22, the World Health Organization said 863 cases of the birth defect had been traced back to Zika infections in Brazil.)

The mosquitoes responsible for spreading Zika are a formidable foe. Each female mosquito can lay about 1,000 eggs in her short lifetime, and she can lay eggs just about anywhere there’s a bit of moisture and warm enough temperatures. Eggs laid on dry surfaces can survive for more than a year, and will wait to hatch until they get wet. Once that happens, if it’s still too cool out for the baby mosquitoes, they can stay in the larval phase for many months more, as long as they have enough water to stay submerged.

Aedes aegypti thrive in crowded cities. They’re known for their sneak attacks—biting people from behind and targeting elbows and ankles, areas where they’re least likely to get swatted away, according to the WHO. They love people, but they aren’t crazy about animals. Meaning, if you live in a place where Aedes aegypti flourish, you are what they want to eat. You and all your friends, that is.

Aedes aegypti are known as sip-feeders because they like to take little glugs of blood from different sources, a practice that increases the likelihood that any single mosquito will transmit disease to multiple people. If they can get into your house, all the better for them. Aedes aegypti like being indoors. They hang out in closets, under beds, and behind washing machines; often emerging around dawn and dusk for blood meals from people who usually won’t even notice they’ve been bitten until it’s too late.

“All of these features make Aedes aegypti populations extremely difficult to control,” the World Health Organization says. “They also make the diseases they spread a much larger menace.”

If there is any good news to be found amid the threat of a Zika outbreak in the United States, it’s that an epidemic is not inevitable. So far, there have been no cases of locally transmitted Zika in the U.S.; all 388 cases of Zika recorded by the Centers for Disease Control and Prevention were brought back to the U.S. by people who had traveled to places where mosquitoes are transmitting the virus. Then again, mosquito season is just getting started. And the true scope of the public-health threat in the States is unknown.

That’s in part because the population of Aedes aegypti may extend much farther north than scientists originally thought. Cities in South Florida and along the Gulf Coast, as well as some areas of Arizona and California, are already known to host large Aedes aegypti populations, and researchers have identified at least 50 metro areas with meteorological conditions that are ideal for the species. But even beyond those areas, the CDC has revised one map to extend the distribution of Aedes aegypti, which in recent years has been found in pockets of Washington, D.C., New York, and even New England.

There’s also the question of the extent to which other mosquito species will be Zika vectors. Aedes albopictus, also known as the Asian tiger mosquito, is even more abundant in the United States than Aedes aegypti. In fact, many entomologists believe the tiger mosquito is quickly replacing Aedes aegypti. (However, one study found the two species often seem to co-exist just fine.)

“But most people believe Aedes albopictus won’t be as efficient [a vector] as Aedes aegypti, because it also feeds on other mammals and birds,” said Peter Hotez, the dean of the National School of Tropical Medicine at Baylor College of Medicine. In other words, if the tiger mosquito is getting a good portion of its blood meals from horses and dogs, that means fewer opportunities for infected mosquitoes to spread disease among humans.

More concerning than the Asian tiger mosquito, Hotez told me, is the possibility that Culex mosquitoes might become effective vectors of Zika in the United States. The distribution of this genus, which is also a vector for West Nile virus, is extensive in the United States, even in cooler regions. Preliminary research has indicated that transmission among Culex mosquitoes is possible in a lab setting, but more study is necessary.

“Transmission of Zika through Culex mosquitos may be possible,” Hotez said. “If that turns out to be the case, then we’re all totally screwed. There are massive numbers of them already. They’re so much more widespread.”

Already, public-health officials are warning about a significant uptick in Zika cases in the months to come. Marie-Paule Kieny, the assistant director of the WHO, said in a press conference on Monday that a combination of local transmission and sexual transmission of the virus poses the risk of a “global emergency.”

“The mosquito knows no border,” she said.