The New England Journal of Medicine reported in early June 2016 that the Zika virus can be transmitted through oral sex. One such case was found in France, when a man returned from Rio de Janeiro and passed the virus on to a partner in this manner.  

While that case remains the only one of its kind that public-health experts are aware of, the finding has not come as a total surprise: The Zika virus is known to live in semen, and it's now clear that the disease—while mainly spread by mosquitoes—can also be transmitted sexually. 

"We did not expect that sexual transmission would be as common as we've seen it," said Thomas R. Frieden, M.D., the director of the Centers for Disease Control and Prevention, at the National Press Club in late May. "We've never had sexual transmission of dengue or West Nile, but Zika can spread sexually. That adds a new level of risk," he said.

While many questions about the virus remain outstanding, the basics of transmission are clear. Here’s what we do and don't know about how Zika spreads from one person to another.

Mosquito Bites: Most Common Mode of Transmission

Here’s how this works: A female Aedes mosquito bites an infected human (only females bite, and so far the virus has not proven capable of being spread through animals). She has to bite during the viremic stage—that is, in the first several days of infection, when the virus is still circulating in the human's blood. The mosquito takes up the virus, stores it in her gut and salivary glands, and then transmits it when she bites another, uninfected human.

Aedes mosquitoes are aggressive daytime biters. They thrive near human habitats and need only very small amounts of standing water to lay their eggs. In addition to properly using insect repellents, one of the best ways to prevent transmission is by removing standing water (from buckets, old tires, animal dishes, flower pots, and so on) from around your home.

Sexual Transmission: More Common Than First Thought

Men—but so far not women—can pass the Zika virus on to their sexual partners during intercourse. The virus actually lives longer in semen (up to two months, and possibly much longer) than it does in blood (a couple of weeks). So far, there have been 11 cases of sexual transmission in the U.S., all of them between men returned from Zika-affected countries and their sexual partners. 

Pregnant Women Can Pass the Virus to Unborn Babies

That can happen either during pregnancy or around the time of birth. The Zika virus is capable of crossing the placenta and infecting the fetus during any trimester (called congenital transmission). Pregnant mothers can also pass the virus at or around the time of delivery (called perinatal transmission).

The virus is particularly dangerous for unborn children because it can cause a devastating birth defect called microcephaly. While microcephaly has become the signature effect of Zika, there are still many unknowns about the relationship. Scientists can’t yet say how likely a pregnant mother is to pass the virus on to her fetus, or how likely the virus is to cause microcephaly and other birth defects. They also don’t know which stage of pregnancy is most vulnerable to the effects of Zika, or how great the risk of other outcomes, like miscarriage is.

Zika Can Also Be Passed Through Blood Transfusions

There have been no cases of this kind in the U.S.. But there have been several reported cases in Brazil, most of them from early on in the outbreak. Epidemiologists at the Centers for Disease Control and Prevention are still looking into these reports. But in the meantime, they are erring on the side of caution: If you’ve recently traveled to a Zika-infected country, the American Red Cross asks that you refrain from donating blood for at least four weeks.

Ways Zika Is Not Transmitted

So far, Zika is not transmitted in the following ways. But things could change, and scientists are learning more everyday about the Zika virus. We'll update you as their understanding evolves.

  • From a woman to man during sex. So far, the virus has not been found in vaginal swabs of infected women, and no cases of an infected woman passing the virus on to a sexual partner has been reported.
  • From an asymptomatic man during sex. Every man who has transmitted the virus through sex has shown symptoms of Zika infection, such as fever, joint pain and mild rash. It's still possible that asymptomatic men can pass the disease. But the lack of such cases is good news; the vast majority of people infected with Zika show no symptoms.
  • Via breast milk. So far there are no examples of infection passing through breast milk, and new mothers in Zika-affected countries are still encouraged to breast-feed.
  • Through kissing. The virus has been found in saliva (as well as urine); but so far there have been no reported instances of it being transmitted through kissing.