The mosquito-borne virus Zika didn’t come out of nowhere, but it kind of feels that way, given how rapidly it’s spread both through the Americas and through public consciousness over the past month or so.

And now, after an emergency committee meeting that met by teleconference on Monday, the World Health Organization declared the outbreak of the Zika virus a “public-health emergency of international concern.” This is a rare designation, previously given to outbreaks like H1N1 in 2009 and Ebola in 2014, as my colleague Krishnadev Calamur reported.

The short version of Zika’s history goes like this: It was identified in 1947 in the Zika forest in Uganda. For decades, it laid low in Africa and Southeast Asia, with just 14 documented cases until 2007, when the first major Zika outbreak hit Yap island in Micronesia. Other Pacific Islands began to see more cases, and in 2013, there was another outbreak in French Polynesia. The current outbreak began in Brazil in May 2015 and has since spread to 22 other countries in North and South America, plus Puerto Rico and the U.S. Virgin Islands, according to the Centers for Disease Control and Prevention. No one knows how Zika got to Brazil in the first place, but Reuters reports that the Brazilian government thinks a traveler to the World Cup may have brought it into the country in 2014.

On Tuesday, the CDC confirmed a case of sexually transmitted Zika in Dallas, Texas, in which a person spread the virus to a sexual partner after contracting it abroad. Prior to this, it was suspected that Zika could be sexually transmitted—traces of the virus were found in a patient’s sperm during the French Polynesia outbreak—but no one knew for sure.

While the outbreak is now growing quickly—the WHO director general, Margaret Chan, declared Zika to be “spreading explosively” through the Americas—the emergency is not so much Zika itself, but the neurological conditions associated with it. Since last May, Brazil has seen an uptick in cases of microcephaly (a birth defect of small head size which can signal brain damage) and Guillain-Barré syndrome (in which the immune system attacks the nervous system). The WHO “strongly suspect[s]” a causal connection between Zika and microcephaly, though the link hasn’t yet been proven; the connection with Guillain-Barré is likewise suspicious but not confirmed.

On its own, Zika isn’t much to worry about—it’s rarely fatal, and is typically milder than its more painful cousin dengue, though the two share symptoms like fever, rashes, and joint pain. Most people who are infected with Zika (four out of five of them) will never experience any symptoms at all. “For most of the non-pregnant population, there is no reason to think Zika presents a particular risk,” the CDC director, Tom Frieden, wrote for CNN.

While this is great news for the casual traveler who just wants to visit Rio and avoid a rash, it makes it difficult to estimate how many people have actually been infected in this outbreak. If you’ve been infected with Zika, chances are you won’t know it. It’s a lurker. And now that we’ve seen it can be transmitted sexually, that's doubly worrying.

Plus, it really complicates the picture for pregnant women. If Zika does cause microcephaly, a lack of Zika symptoms makes it hard to know who’s at risk for having a baby with the birth defect.

But the number of cases of microcephaly in Brazil may be a little overblown—the Brazilian health ministry has reported around 4,000 suspected cases since October 2015, but these are just suspected, not confirmed. Some of those babies may just have small heads, and no developmental problems. The Global Post reports that the number of confirmed cases is much lower:

So far, of the original thousands of suspected cases, Brazil has confirmed 270 instances of children born with microcephaly since October 2015. And, significantly, 462 cases have been discarded, according to the ministry’s numbers. So cases are being discarded at almost twice the rate that they’re being confirmed.

“When you’re studying something in epidemiology, there's always going to be a little bit of over-reporting compared to the confirmed number of cases of a condition,” says Chad Achenbach, an infectious-disease specialist at Northwestern Memorial Hospital and Northwestern University. But even if some of those suspected cases don’t pan out, the increase is still notable—from 2010 to 2014, microcephaly cases in Brazil were hovering around 150 per year.

“What you need to ask yourself when you see something like this is, ‘Is there something else that could've explained this increase besides Zika?’” Achenbach explains. “You have to be careful with making those types of associations, but this is such a big increase and there doesn’t seem to be any other particular reason.”

“This is kind of an unprecedented outcome of this type of infection,” he says. “We’ve never seen this type of birth defect with similar types of viruses.” Not even with West Nile, which can cause neurological problems—like meningitis and encephalitis—in adults.

Zika is spread by two species of mosquitoes: Aedes aegypti and Aedes albopictus. These same mosquitoes can also transmit West Nile, dengue, and chikungunya—all diseases that have made inroads in the Americas in recent years.

“It is not surprising that [Zika] has followed a similar path of rapid expansion upon arriving in the Americas,” Jane Messina, a senior postdoctoral epidemiologist at the University of Oxford, told me in an email. “Ae. aegypti are abundant in densely populated urban areas of this region, and so the populations in these areas, having no background immunity to Zika, are highly susceptible to infection.”

Anthony Fauci, the director of the National Institute of Allergy and Infectious Diseases, calls this “the perfect storm of an outbreak of a new mosquito-borne infection.”

Zika’s place on this family tree is complicated—it looks very similar to dengue and chikungunya, and because there is no commercially available diagnostic test for Zika, it can be hard to tell which one someone has. (Right now, testing for Zika can only be done at the CDC Arbovirus Diagnostic Laboratory and some state health departments.) But it’s important—not just because of Zika’s connections to microcephaly and Guillain-Barré, but also because dengue “is a bit more severe disease, requiring a bit more supportive care and monitoring,” Achenbach says. In some people (particularly those who’ve had dengue before), it can develop into the serious dengue hemorrhagic fever. What’s more, dengue, chikungunya, and West Nile are not sexually transmissible, so clarifying that someone has Zika is also clarifying whether they need to use condoms and protect their sexual partners.

There are no currently available vaccines or treatments for Zika. (Fauci says the NIAID hopes to have a vaccine in a Phase One clinical trial by the end of the summer.) The official CDC recommendation is just to protect yourself from mosquito bites if you travel to affected countries, and to avoid contact with the semen of someone who’s been to those countries.

Pregnant women or women who are trying to get pregnant are advised to delay travel, or at least talk to their doctors before going. (This is a comparatively chill recommendation—as my colleague Uri Friedman reported, some countries with local transmission of Zika have advised women to put off having kids altogether.) If pregnant women returning to the U.S. from affected countries have Zika symptoms, the CDC recommends they be tested, with possible amniocentesis to check on the fetus if the results are positive. If they don’t have symptoms, the agency still recommends at least an ultrasound. (There’s a whole flowchart here.)

Up until now, the term “local transmission” has been used to mean that Zika is being spread via mosquitoes within a country. Achenbach says that sexual transmission also counts as local transmission, but it’s unclear whether the new Dallas case will prompt the CDC to add the United States to its list of countries included in the outbreak. (I have asked the agency this question, but have yet to receive a response.) Regarding adding the U.S. to the list, Fauci says, “I think with a single case of sexual transmission I wouldn’t go that far.”

But it definitely shakes things up. “I think we have to reevaluate the whole concept of what local transmission is and what the role of returning travelers are," Fauci continues. “Because before, returning travelers would be considered as imported cases. But if the imported case has the potential of spreading [Zika], then we have to figure out ways that we can prevent that spread. So all of a sudden it adds a whole new dimension to what we mean by an imported case.”

But the U.S. probably will, at some point, see transmission by mosquitoes as well. According to a model recently published in The Lancet, 22.7 million Americans live in regions of the U.S. where Zika could spread year-round, and 200 million, or 60 percent of the population, live in places where it could spread during warmer months.

Other than the sexually transmitted case, all the Zika cases in the U.S. so far have been in travelers bringing it home as an unwanted souvenir. A student at the College of William and Mary in Virginia recently contracted the virus after vacationing in Central America, and a handful of others have similar stories, including a woman in Hawaii whose baby was born with microcephaly and tested positive for Zika. The mother had lived in Brazil for a while last year.

No one in the U.S. has yet contracted the virus through a bite from an American mosquito. For that to happen, the right kind of mosquito would have to bite an infected person who brought the virus from another country, become infected itself, then bite and infect someone else. Or, Achenbach says, an infected mosquito could hitch a ride on an airplane from another country and buzz off into the American sunset.

Achenbach says he thinks it’s likely that there will be some transmission from mosquitoes in the U.S., and possibly some “very limited small outbreaks, most likely in Southern states—Florida, Louisiana, Mississippi, Texas.” But the virus should stay fairly contained, partially because cities in these areas tend to be less population-dense than cities in Central and South America, partially because “our mosquito-control programs are a little bit better,” Achenbach says, and partially because so many people have window screens and air conditioning. The A/C helps because mosquitoes don’t like cool temperatures, and neither does Zika—“the ability of the virus to replicate within the mosquito [is] hindered by lower temperatures,” Achenbach says.

Despite all the unanswered questions and lack of treatments and vaccines, according to Fauci, the key to slowing and stopping the outbreak is old-fashioned mosquito control—insecticides and removing standing water. “How long the outbreak goes on depends on how well you control the mosquitoes,” he says.

Zika’s sudden appearance and rapid rise in the Western hemisphere likely owes a lot to both urbanization and international travel—when people cluster and move around, so do viruses. (The same can be said for Zika’s cousins in the region.) And a warming planet means there are more areas where these mosquitoes can thrive. However long this outbreak goes on, mosquito-borne viruses are likely here to stay in North and South America.

“Given that controlling the Ae. aegypti population and spread of other mosquito-borne viruses has been such a struggle in this region,” Messina says, “I don’t see any reason why Zika should now disappear from the Americas.”