The science on Zika is changing rapidly. Here’s what we’ve learned to date.

Zika's rapid spread around the world has quickly changed our scientific understanding of the virus.

Disease outbreaks are terrible for people but great for our scientific understanding of viruses. That's especially true for the Zika virus, which seemingly burst out of nowhere to become a top health research priority in the past year.

Prior to the massive, ongoing outbreak in the Americas, there had only been about 25 published research papers on the disease since 1952. Researchers long thought Zika was a fairly benign virus, spread mainly by mosquitoes and endemic to Africa. It wasn't on the radar as any sort of threat. It wasn't even a disease health officials in the US bothered to track.

Since Zika was identified in Brazil in March 2015, however, the virus has infected more than a million people there. It has spiraled outward to become the world's largest Zika outbreak ever, reaching more than 30 countries and territories in the Americas and most recently popping up in Vietnam.

Along with the epidemic, there's been a rush of new knowledge generated about the disease. There are now 313 Zika articles on PubMed — and 225 of them were published in 2016 alone, according to the New England Journal of Medicine. Here are some of the most important nuggets of what researchers have recently learned.

1) Scientists finally know what Zika looks like

Researchers have long known that Zika is a flavivirus, in the same family as dengue, yellow fever, and West Nile. But they didn't know exactly what Zika's structure looked like. For the first time this year, researchers at Purdue University mapped it out (and shared the results in the journal Science). Using a technique called cryo-electron microscopy, the researchers could image the virus much more quickly and in much higher resolution than they could have with traditional X-ray crystallography.

The finding is expected to help researchers develop a Zika vaccine or cure. "The structure of the virus provides a map that shows potential regions of the virus that could be targeted by a therapeutic treatment," said one of the researchers on the paper, Purdue's Richard Kuhn, in a statement.

2) Zika can be a sexually transmitted disease

At the beginning of this outbreak, researchers thought Zika was transmitted by mosquitoes and maybe, just maybe, through sex in rare cases.

Now researchers still think mosquitoes are the primary Zika spreaders, but they've learned Zika can definitely be sexually transmitted and that the virus is spread through sex more commonly than they thought. (It can also be transmitted from pregnant mothers to their fetuses — more on that below.)

Adding to a handful of cases in the medical literature from past Zika outbreaks, researchers and public health officials have documented many more cases of sexual transmission over the past year — in the US, Chile, France, and New Zealand, among other spots. These cases are usually identified when a traveler returns from a country where Zika has been circulating and then gives the virus to his partner.

There's a lot researchers have to learn about this mode of transmission. They aren't sure how long Zika can remain in semen, whether the virus can be spread through oral sex, or whether women can pass the virus through sex (so far, cases have involved men spreading the virus).

For now, the Centers for Disease Control and Prevention issued guidance on safe sex during a Zika outbreak, suggesting men who have traveled in Zika countries and have female partners who are pregnant either avoid sex or use condoms for vaginal, anal, and oral sex during the pregnancy. For couples who aren't pregnant, the CDC recommends considering condoms or abstaining, regardless of whether either person was infected with the virus.

3) There's more strong evidence that Zika can be incredibly damaging to fetuses

(New England Journal of Medicine)

According to the World Health Organization, there's now strong evidence that Zika is linked to a terrible birth defect called microcephaly, which is characterized by a shrunken head and incomplete brain development.

The association between the condition and Zika has been seen in this outbreak in Brazil, as well as in reanalyses of the data from previous outbreaks, such as one in the French Polynesian islands in 2013-'14. Other countries, including Cabo Verde, Colombia, Martinique, and Panama, have also reported microcephaly or other fetal malformations to the WHO over the last year.

In addition to microcephaly, researchers have found that the virus seems to kill off the tissue in entire regions of the brain, damage babies' eyes, and heighten the risk of miscarriage and fetal death.

Researchers believe pregnant women are at the greatest risk of having babies with birth defects if they are infected in the first trimester. They've also estimated that women infected with Zika during the first trimester of their pregnancies face a 1 in 100 chance of delivering a baby with microcephaly.

Zika wouldn't be the first virus to cause microcephaly. Rubella famously caused an epidemic of birth defects before the advent of the vaccine, and cytomegalovirus (CMV) can damage fetuses too. But Zika is the first mosquito-borne virus to threaten fetuses. And even if birth defects turn out to be a very rare complication of Zika, the risk to fetuses was enough to prompt the WHO to declare a rare global public health emergency earlier this year.

4) Zika causes neurological complications

Following Zika's emergence in a country, researchers have documented an association between the virus and Guillain-Barré, a rare and sometimes deadly neurological condition in which people's immune systems damage their nerve cells, leading to muscle weakness and even paralysis. The symptoms can last weeks, months, or even years.

According to the WHO, 13 countries or territories battling Zika outbreaks have noted an increased incidence of Guillain-Barré.

There are also reports of other neurological complications related to the virus: Zika and meningoencephalitis, as well as acute myelitis, dangerous inflammations of the brain and spinal cord brought on by an infection.

5) The virus probably didn't come to Brazil with the World Cup

Since the outbreak in Brazil began, researchers speculated that it originated in 2015 with travelers arriving for the World Cup or the Va’a canoe event held in Rio de Janeiro.

But a recently published study, mapping the genomes of the virus from several Brazilian Zika patients against travel patterns in the country, suggests the virus was likely introduced between May and December 2013, much earlier than was previously thought.

"The estimated date of origin coincides with an increase in air passengers to Brazil from ZIKV endemic areas, and with reported outbreaks in Pacific Islands," the researchers wrote in the journal Science. Since Brazil only detected the virus in March 2015, this means it was circulating for about two years, unbeknownst to health officials.

6) A pretty big area of the US is at risk of outbreaks — but no one expects an epidemic

The estimated range of Aedes aegypti and Aedes albopictus moquitoes in the United States. (CDC)

In March, the CDC released an updated map that showed where the mosquitoes that carry Zika dwell — suggesting the virus could spread to a much broader area of the US than previously estimated. (The Aedes aegypti mosquito is believed to be the main Zika carrier since it's adapted to live around and feed off people, though the mosquito that's more common in the US, Aedes albopictus, has also been shown to spread the virus.)

Public health officials are warning that mosquito control strategies should ramp up as temperatures across the country rise and mosquito populations grow, increasing the risk of the virus's spread.

They aren't, however, expecting an epidemic in the US. There have been more than 300 Zika cases involving travelers, but the virus has not been transmitted locally in the continental US. And any future outbreaks are expected to be small and localized.

The main reason: Like many viruses, Zika thrives in conditions of poverty and broken health systems. "Better housing construction, regular use of air conditioning, use of window screens and door screens and state and local mosquito control efforts helped to eliminate [mosquito-borne infections like malaria] from the mainland," said Lyle Petersen, director of the CDC's division of vector-borne diseases, in a recent statement.

These conditions will likely protect Americans, but they aren't present in many developing countries, which suggests Zika could persist around the world for a long time to come.