Editor's note: This story has been updated since it was originally published on Jan. 29.

Zika virus was an obscure illness until recently, especially in the Americas. But now it's "spreading explosively" through the New World, according to the World Health Organization (WHO), which has declared Zika a global public health emergency.

"Most of what we've learned is not reassuring," said Dr. Anne Schuchat, principal deputy director of the U.S. Centers for Disease Control and Prevention (CDC), in an April 2016 statement about the disease. "Everything we look at with this virus seems to be a bit scarier than we initially thought."

Zika could infect as many as 4 million people before the end of 2016, according to WHO director-general Margaret Chan, who says it has gone "from a mild threat to one of alarming proportions." Although Zika has been known to science for seven decades, it's only now recognized as a public-health crisis due to newfound links with birth defects and other potentially severe health problems.

The suddenness of Zika's American outbreak has left many governments scrambling to catch up — and many people wondering what they can do to avoid the virus. Here are brief answers to some of the most vital, and vexing, questions about Zika:

Uganda forestThe Zika virus was discovered in a small Ugandan forest nearly 70 years ago. (Photo: Rod Waddington/Flickr)

Where did Zika come from?

Zika is a flavivirus related to the viruses behind yellow fever, dengue, West Nile and Japanese encephalitis. It's named after Zika Forest in Uganda, where it was first identified in a monkey in 1947. It's endemic to Africa, and likely spread to Asia 50 years ago, but for decades only sporadic human cases were reported.

The first documented outbreak of Zika occurred in 2007, on Yap Island in the Federated States of Micronesia. The virus then spread to other South Pacific islands, aided by the lack of immunity among people in that region. It was first confirmed in Brazil in May 2015, and has since spread rapidly through the country — as well as to more than 40 other countries and territories.

Aedes aegypti mosquitoAedes aegypti mosquitoes are the main vector for Zika virus. (Photo: Luis Robayo/AFP/Getty Images)

How does it spread?

Zika is transmitted by infected Aedes mosquitoes like Aedes aegypti and Aedes albopictus, also known as the Asian tiger mosquito. Those two species evolved in Africa and Asia, respectively, but they're now common in warm, wet climates worldwide. They often live near buildings in urban areas and are usually active during the day, with peak biting periods in early morning and late afternoon. Both species can transmit Zika, yet while Ae. albopictus feeds on a variety of mammals and birds, Ae. aegypti is considered a bigger threat because it specializes in biting humans.

Despite Zika's reliance on mosquitoes, the virus is capable of spreading in other ways, too. It can jump from an infected mother to her baby during pregnancy or around the time of birth, for example, although it's not clear how frequently that happens. The virus can also spread via blood transfusions and sex; the latter method was responsible for the first U.S. case of Zika transmission, and several others since.

What are the symptoms?

Only about one in five people infected with Zika virus will get sick, according to the CDC, and even then it may seem mild. Common symptoms include a low-grade fever, rash, joint pain, headache and conjunctivitis. The incubation period is typically two to seven days, and the prevailing wisdom has been that any symptoms should appear within about a week of being bitten by an infected mosquito. According to a recent study of Zika in monkeys, however, pregnancy "may drastically prolong the time the virus stays in the body," from 10 days to as much as two months.

Zika is also associated with Guillain-Barré syndrome, an immune disorder than can cause paralysis. A study published in February 2016 strengthened that link, finding that 39 people in French Polynesia developed Guillain-Barré within about six days after a Zika infection. That makes it "the first study providing evidence for Zika virus infection causing Guillain-Barré syndrome," its authors write in the Lancet.

Another study has linked Zika with acute disseminated encephalomyelitis (ADEM), which is characterized by a brief but widespread attack of inflammation in the brain and spinal cord that damages myelin, the protective covering of nerve fibers.

And according to another troubling study published in March, Zika seems to preferentially kill developing brain cells. The virus infected 85 percent of neural stem cells within three days in a lab setting, first "hijacking" their cellular machinery to replicate itself before eventually killing them. This could help explain how Zika triggers its most feared health problem: brain defects in babies.

How does Zika affect babies?

microcephaly and Zika virusAfter Zika's 2015 arrival in Brazil, doctors began to notice an uptick in cases of microcephaly, a birth defect characterized by an undersized head and incomplete brain development. Brazil typically has about 150 reports of microcephaly per year, but the country is now investigating nearly 4,000 cases from 2015 — 20 times the normal amount. The connection wasn't widely reported until October, and since then the prospect of microcephaly has made Zika as dreaded as much deadlier diseases.

This association still isn't well-understood, and for months scientists couldn't say with certainty whether Zika causes microcephaly. Microcephaly can also be triggered by a wide range of genetic, environmental or infectious factors, although scientists now say they are confident that a Zika infection can cause microcephaly in unborn babies.

Why weren't these health risks noticed much earlier, in Africa or Asia? That's still a bit of a mystery, but it could be due to greater immunity in those populations than in the Americas, where the virus is a newcomer. Zika's previously mild reputation also inspired less scrutiny than other insect-borne diseases, so its worst effects may have simply gone unnoticed. Plus, a recent study found the virus is capable of mutating rapidly, and "has undergone significant genetic changes in the past 70 years."

In addition to microcephaly, the CDC warns of other birth defects consistent with Zika like intracranial calcifications or brain and eye abnormalities, which may be harder to detect via fetal ultrasound until the late second or early third trimester.

active Zika virus transmission This map shows countries and territories with active Zika virus transmission, as of May 19, 2016. (Image: CDC)

What countries have Zika?

Zika is expected to eventually spread locally in every American country except two: Canada and Chile, whose climates are less conducive to Aedes aegypti. There is now active Zika virus transmission in the following places, according to the CDC:

  • American Samoa
  • Argentina
  • Aruba
  • Barbados
  • Belize
  • Bolivia
  • Bonaire
  • Brazil
  • Cape Verde
  • Colombia
  • Costa Rica
  • Cuba
  • Curaçao
  • Dominica
  • Dominican Republic
  • Ecuador
  • El Salvador
  • Fiji
  • French Guiana
  • Guadeloupe
  • Guatemala
  • Guyana
  • Haiti
  • Honduras
  • Jamaica
  • Kosrae, Federated States of Micronesia
  • Marshall Islands
  • Martinique
  • Mexico
  • New Caledonia
  • Nicaragua
  • Panama
  • Papua New Guinea
  • Paraguay
  • Peru
  • Puerto Rico
  • Saint Barthélemy
  • Saint Lucia
  • Saint Martin
  • Saint Vincent and the Grenadines
  • Sint Maarten
  • Samoa
  • Suriname
  • Tonga
  • Trinidad and Tobago
  • U.S. Virgin Islands
  • Venezuela

Because of the potential risk to babies, the CDC recommends that women who are pregnant, in any trimester, "consider postponing travel to any area where Zika virus transmission is ongoing." If you must travel to one of these places, the CDC advises consulting a doctor first and strictly following steps to avoid mosquito bites.

And due to the possibility of Zika spreading through blood transfusions, the American Red Cross issued a statement in March asking people to avoid donating blood for at least 28 days after traveling to Mexico, the Caribbean, Central or South America. If you develop Zika-like symptoms within 14 days of donating blood, notify the Red Cross immediately so it can quarantine the blood.

Where do Zika mosquitoes live?

The mosquitoes capable of transmitting Zika are abundant in tropical and subtropical regions around the world, as well as many temperate areas. Aedes albopictus has adapted to survive at cooler temperatures, which lets it inhabit a wider range with more variable climates. It lives in close proximity to people, but less so than Aedes aegypti. This map shows the global distribution of both species.

The maps below, published this month in the Journal of Medical Entomology, show the rising reports of Ae. aegypti and albopictus, respectively, by U.S. county since 1995:

Aedes aegypti U.S. map Reported occurrence of Aedes aegypti by U.S. county from January 1995 to March 2016. (Maps: CDC)

Aedes albopictus U.S. map Reported occurrence of Aedes albopictus by U.S. county from January 1995 to March 2016. (Map: CDC)

For more about Zika risk in the U.S., see these maps from the CDC.

Is there a vaccine or a cure?

Not yet. One vaccine has been approved for human testing by the U.S. Food and Drug Administration (FDA), and several others are in various stages of development. For now, however, there is no vaccine or specific medicine for Zika virus infections, and the CDC doesn't recommend any particular antiviral treatment. Doctors can only provide supportive care and treat symptoms. Patients should get plenty of rest, stay hydrated and possibly take acetaminophen for pain or fever, but should avoid aspirin and other non-steroidal anti-inflammatory drugs (NSAIDs).

Initial trials of the first Zika vaccine are slated for late 2016. If those go well, larger trials could occur in 2017. Even in the best scenario, however, experts say a vaccine won't be widely available until at least 2018. (On the bright side, the authors of one recent study say their findings "provide substantial optimism that the development of a safe and effective Zika virus vaccine for humans will likely be feasible.")

In the meantime, researchers are continuing to probe Zika for potential weaknesses. A recent study from the University of Massachusetts, for example, found that a human protein known as interferon-induced protein 3 (IFITM3) can "inhibit Zika virus infection early in the viral life cycle," the authors write, and "prevent Zika-virus-induced cell death." This is still an early discovery with unclear implications, but it's a welcome note of optimism for an outbreak that has grown increasingly bleak.

"This work represents the first look at how our cells defend themselves against Zika virus' attack," explains senior author Abraham Brass. "Our results show that Zika virus has a weakness that we could potentially exploit to prevent or stop infection."

Is there a test to diagnose it?

There are no commercially available diagnostic tests for Zika, according to the CDC, and its close relation to dengue and yellow fever makes it prone to cross-reaction with antibody tests for those flaviviruses. Testing can be done at the CDC Arbovirus Diagnostic Laboratory, however, and a few state health departments.

If a pregnant woman may have been exposed to Zika, what should she do?

The CDC issued interim guidelines for this scenario in early April, although like much of the agency's advice so far, it's subject to change as more information comes in.

For pregnant women who might have been exposed to Zika, the CDC has two interim guides — one for women who don't live in an area with active Zika transmission, and one for women who do. "[A] decision to implement testing of asymptomatic pregnant women should be made by local health officials based on information about levels of Zika virus transmission and laboratory capacity," the agency adds.

What if someone becomes pregnant after visiting an affected country?

"Women who have Zika virus disease should wait at least 8 weeks after symptom onset to attempt conception, and men with Zika virus disease should wait at least 6 months after symptom onset to attempt conception," the CDC says. "Women and men with possible exposure to Zika virus but without clinical illness consistent with Zika virus disease should wait at least 8 weeks after exposure to attempt conception."

Based on the evidence so far, the CDC believes a Zika infection in a woman who isn't pregnant likely wouldn't pose a risk of birth defects in future pregnancies, as long as the virus has cleared from her blood. "From what we know about similar infections," it adds, "once a person has been infected with Zika virus, he or she is likely to be protected from a future Zika infection."

stagnant water and Zika virus A soldier takes a water sample in Realengo, Brazil, looking for Aedes aegypti larvae. (Photo: Antonio Scorza/AFP/Getty Images)

Is climate change involved?

Like weather, it's hard to link any specific disease outbreak to broad climatic trends. But as with storms and droughts, climate change is expected to favor certain insect-borne disease like Zika by favoring the mosquitoes that spread them.

Climate change may not be to blame for Zika reaching the Americas, but by expanding the tropical and subtropical biomes where Aedes mosquitoes thrive, it could worsen the disease now that it's here. All mosquito-borne diseases rely on their hosts to get around, so their fate hinges on things that affect mosquito numbers — namely density of people, amount of rain and length of summer in a given area. Rising temperatures extend the breeding season for mosquitoes, which can also mature and reproduce more rapidly in warmer, wetter weather.

As Chelsea Harvey reports in the Washington Post, however, the relationship between climate and mosquito-borne diseases isn't necessarily as simple as it seems. "With warming, we change our behavior," University of Arizona epidemiologist Heidi Brown tells Harvey. "We might go out earlier, we might go out at different times of the day. People working outdoors, people not working outdoors, whether we're running air conditioning and staying indoors — it's a combination of these things."

Zika in El SalvadorA worker sprays for mosquitoes in El Salvador, where the government's Zika response includes advising women to avoid pregnancy for the next two years. (Photo: Marvin Recinos/AFP/Getty Images)

What can be done to stop it?

Scientists and health officials around the world are racing to answer this question, both by trying to defeat Zika itself and by trying to predict its spread. Private companies are also pitching in — Google, for one, has donated $1 million to UNICEF's anti-Zika efforts and set up a volunteer team of engineers, designers and data scientists to help UNICEF "build a platform to process data from different sources (i.e., weather and travel patterns) in order to visualize potential outbreaks."

According to one recent study, our efforts to control Zika may be less effective than the virus's own limitations. "Our analysis suggests that Zika spread is not containable, but that the epidemic will burn itself out within 2-3 years," lead author and Imperial College London researcher Neil Ferguson says in a statement.

While the quest for a vaccine or treatment continues, the best way to avoid Zika is to avoid places with active transmission. That doesn't help people who live in those places, however, and it may just be a matter of time before the virus becomes established in parts of the Southern U.S. "The best way to prevent diseases spread by mosquitoes," the CDC points out, "is to avoid being bitten."

The first step to limit mosquito bites is to limit mosquitoes, which means limiting the stagnant water where they breed. (Researchers are still studying more drastic measures, like releasing genetically modified mosquitoes to curb wild populations.)

Here are more tips from the CDC for avoiding Zika (and similar diseases):

  • Wear long-sleeved shirts and long pants.
  • Stay in places with air conditioning or that use window and door screens to keep mosquitoes outside.
  • Use EPA-registered insect repellents. All EPA-registered insect repellents are evaluated for effectiveness.
  • Always follow the product label instructions.
  • Reapply insect repellent as directed.
  • Do not spray repellent on the skin under clothing.
  • If you are also using sunscreen, apply sunscreen before applying insect repellent.
  • Do not use insect repellent on babies younger than 2 months of age.
  • Dress your child in clothing that covers arms and legs, or ...
  • Cover crib, stroller, and baby carrier with mosquito netting.
  • Do not apply insect repellent onto a child's hands, eyes, mouth or irritated skin.
  • Adults: Spray insect repellent onto your hands and then apply to a child's face.
  • Treat clothing and gear with permethrin or purchase permethrin-treated items.
  • Treated clothing remains protective after multiple washings. See product information to learn how long the protection will last.
  • If treating items yourself, follow the product instructions carefully.
  • Do not use permethrin products directly on skin. They're intended for clothing.
  • Sleep under a mosquito bed net if you're overseas or outside and can't protect yourself from mosquito bites.

Russell McLendon ( @russmclendon ) writes about humans and other wildlife.