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The CDC is warning about sexually transmitted Zika as more possible US cases pop up

As researchers learn more about Zika, they're answering some very basic questions about the virus — such as how exactly it spreads.

Evidence is mounting that it's not just mosquitoes that can pass the disease from person to person: Zika can be spread through sex, too.

The Centers for Disease Control and Prevention just announced it's investigating 14 cases of sexual transmission of Zika in the United States. The cases all involved male travelers who recently visited a region with a Zika outbreak and then returned to the United States, where they reportedly passed on the virus to their female partners.

Several of the cases include pregnant women, which is worrisome considering Zika has been linked with a birth defect called microcephaly. All together, the CDC suggested sexually transmitted Zika may be more common than researchers previously believed.

Researchers have a lot of questions about sexually transmitted Zika

There have already been several cases in the medical literature that suggest Zika can be spread through sex.

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 found in semen.

The first case of sexual transmission in the United States during this current outbreak was documented in Dallas in February. A traveler returned from Venezuela, where the virus was circulating, and infected his or her partner.

But there's still 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. They also don't know how commonly Zika is spread sexually as compared with mosquito bites, though they think it's relatively rare.

Sexual transmission is also thought to spread only from men to women (not vice versa). Again, that information might change as we learn more.

Men coming from Zika-affected regions should avoid unprotected sex with pregnant partners

For now, the CDC has issued interim guidance on safe sex during a Zika outbreak: Men who have traveled or lived in countries where Zika is circulating and have female partners who are pregnant should either avoid sex or use condoms for vaginal, anal, and oral sex for the duration of the pregnancy.

For couples who aren't pregnant, the CDC still recommends considering condoms or abstaining if the male has traveled or lived in a place with Zika.

"The science is not clear on how long the risk should be avoided," the CDC said in its announcement. That's because researchers don't know how long the virus can live in the semen — and how long it remains infectious. In the UK, researchers found virus in a man's semen 62 days after he got sick.

Pregnant women have been advised to avoid Zika hot spots

Meanwhile, American women who are pregnant have been advised to stay out of countries where the Zika virus is circulating. The CDC also issued guidance on how to care for pregnant women during a Zika outbreak, safe sex, and when to get tested for Zika. (See our graphic above.)

If you're trying to get pregnant, proceed with caution if visiting these areas. The CDC recommends consulting your doctor before your trip and following steps to prevent mosquito bites during the trip.

If you'd like to get pregnant in the more distant future, however, there appears to be no need to worry. Zika does not seem to pose a risk of birth defects for future pregnancies. As best researchers can tell, the virus clears itself from the body pretty quickly, remaining in the blood for only about a week after infection. (If you want more information, see our Zika pregnancy explainer.)

Mosquitoes are still thought to be the main disease vector

Right now, researchers believe you're most likely to get Zika through a mosquito bite. The virus 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.)

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, though researchers think this mosquito doesn't spread the virus as efficiently.

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