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All the ways Zika can spread, ranked by scientific certainty

The Zika virus that's currently spreading throughout Latin America and the Caribbean is mostly transmitted by mosquitoes. But health officials just recorded a case in Dallas in which Zika was sexually transmitted.

That has led to a lot of confusion about how Zika actually spreads. And the reality is that scientists are still learning. Up until it arrived in Brazil last year, Zika had never been a major problem in the Western Hemisphere. It didn't seem to harm the vast majority of people who were infected. Now it's affected more than a million people and counting, and it's been linked to a debilitating birth defect called microcephaly. So there's a lot to learn. Here's what we know so far about how Zika can spread:

1) You're most likely to get Zika through a mosquito bite: For now, scientists believe that Zika is mainly carried by a specific type of mosquito called Aedes aegypti, which spreads the disease through bites.

Aedes mosquitoes bite during the day and are incredibly well adapted to thrive alongside humans. They can breed and rest in small pools of water and moist environments around people's homes and apartments. (You can see examples of the mosquito's main aquatic habitats here, ranging from rain-filled cavities in trees to outdoor pots and animal drinking pans.)

(Javier Zarracina/Vox)

There's also some evidence that suggests the Asian tiger mosquito (Aedes albopictus) can transmit the virus, too. This is a worry because the Aedes albopictus has a much larger range in the United States, reaching at least 32 states.

2) Zika can be passed on from moms to fetuses — though we don't know how often this happens: Ever since Zika arrived in Brazil in 2015, doctors in the country have noticed there's been an uptick in birth complications.

ZIKA_VIRUS_MICROCEPHALY Javier Zarracina/Vox

When they studied the amniotic fluid in pregnant women carrying babies with microcephaly (a condition that causes babies' brains and heads to stop growing), they found Zika. Scientists at the Centers for Disease Control and Prevention also found Zika in the brains of two babies with the condition who died within 24 hours of being born.

So researchers are quite certain this is one way Zika can spread, but they have no idea how common this kind of transmission is or exactly when babies are most at risk. This is why the CDC recommends pregnant women avoid traveling in places where the virus is circulating.

3) Zika appears to be sexually transmitted, although this seems to be more rare: Sexual transmission is thought to be a rare but plausible route for spreading Zika.

There have been several cases in the medical literature that suggest this possibility. In one case, a man who traveled to Senegal in 2008 and contracted Zika gave it to his wife through intercourse after he returned home to Colorado. In another, Zika was isolated from semen. The first case of sexual transmission during the current outbreak in the US was documented in Dallas in February: A traveler returned from Venezuela, where the virus was circulating, and infected his or her partner.

There's a lot of uncertainty here. Researchers aren't sure how long Zika can remain in semen, when people are most at risk of passing on the virus, or what types of sex acts are more likely to spread the virus. Sexual transmission is also thought to spread only from men to women (not vice versa), but that information might change as we learn about more cases.

For now, the CDC has issued interim 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 if the male has traveled in a place with Zika.)

4) Zika could be spread through blood transfusions: In Brazil, health officials recently confirmed that the virus can be passed on through blood transfusions. (Earlier research from a Zika outbreak in French Polynesia suggested donors who were asymptomatic tested positive for the virus — but there were no documented cases of infection in the recipients.)

Zika clears from the blood in about a week, but some countries including the US — have asked people who have recently traveled in places with Zika to abstain from donating their blood for about a month.

5) It's possible Zika could be spread through saliva or urine, but we don't know for sure: Finally, there is new — but very limited research — out of Brazil that found the virus in saliva and urine samples of two people infected with Zika. The researchers tested these bodily fluids in people with Zika and found active virus. (This isn't the first time Zika has been found in urine or in saliva.)

Still, there's very little that's known about how infectious these bodily fluids are and whether or how they could infect others. One of the researchers behind the discovery cautioned, "[This] does not mean there is a capacity for transmission through saliva and urine."

Why there's still so much uncertainty around Zika

Before Zika hit Brazil last year, sparking a major outbreak, it hardly bothered humans. There were only about a dozen cases ever recorded before 2007. Now Zika seems to be everywhere, so researchers are learning more about what the virus can do as it affects more people.

One of the things that makes Zika very difficult to track and study is the fact that in the vast majority of cases, the virus causes no symptoms at all. Most people who get infected don't even realize it — and therefore never seek medical attention. They can, however, still transmit the disease if they are bitten by a mosquito that then bites someone else.

For now, Zika seems to be pretty benign in the people who do show symptoms. A minority of Zika patients — roughly 20 percent — show relatively minor symptoms: a low-grade fever, sore body, and headache, as well as red eyes and a body rash. More rarely, this might include abdominal pain, nausea, and diarrhea. These symptoms usually appear two to 12 days after a bite and go away within a week. Severe disease requiring hospitalization is uncommon.

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