Medical Examiner


Your pressing questions, answered.

Pregnant woman in Recife, Brazil
Eritania Maria, who is six months pregnant, is seen in front of her house at a slum in Recife, Brazil, on Feb. 2. Brazil’s poor are worst affected by the mosquito-borne Zika virus.

Ueslei Marcelino/Reuters


So, Zika is sexy now?

No, not exactly. But there is increasing evidence that the mosquito-borne Zika virus can be transmitted via sexual contact between people if a male partner is infected. The U.S. Centers for Disease Control and Prevention has issued new suggestions to prevent this.

Wait, so is Zika sexually transmitted or mosquito-transmitted?

Both. Specific species of mosquitoes can pick the virus up in blood if they bite an infected person. They can then drop it off in the next person they bite. The virus can multiply in the bloodstream but can also get into semen. For unknown reasons, it can be detected in that particular bodily fluid well after the initial infection subsides and can no longer be detected in the bloodstream.


Semen you say. Has anyone actually gotten infected from semen?

Yes. It is rare so far, but the CDC thinks six people in the U.S. have become infected this way, with the first instance occurring in Houston in early February. This is definitively the less-common means of infection—it pales in comparison to the to 273 travel-associated infections in 35 states and 282 locally acquired infections in U.S. territories, mostly in Puerto Rico, at last count.

How do they know for sure it was semen and not mosquitoes?

The newly infected partners were never in an area that is home to the mosquitoes but did have unprotected sex with men who were. A case reported in the scientific literature in 2011 first suggested the possibility of sexual Zika transmission. In another case, a man in Tahiti had the virus in his semen after infection, even though the virus was cleared from his blood.


Are the symptoms the same whether you get Zika from a bug or a dude?

Apparently so. Symptoms are similar to other tropical infections like dengue or chikungunya, and include a skin rash, fever, red eyes, and aches. Symptoms usually last less than 10 days, but up to three-quarters of infections are asymptomatic, which is part of what makes this virus so tricky. Infection has also been linked to microcephaly in babies who were exposed while their mothers were pregnant as well as to a neurological disorder called Guillain-Barré syndrome in adults.

How long does Zika hang around in semen?

Available data say it can be detected for up to two months following infection, while it’s only detectable for about a week in blood samples.


If an infected man gives it to a sex partner, and if that partner is a woman who is pregnant or may become pregnant soon, the fetus could then be at risk of becoming infected as well and possibly developing birth defects.

Can men give it to men via semen? Or can women give it to men, or to other women? Or is this, like, a totally heteropatriarchy-compliant virus?

At the moment the semen transmission route is established from men to women, specifically for men who had the infection and also had symptoms. Whether men who were infected but never had full-blown symptoms can transmit Zika in semen is not know. It is also not yet known whether women can transmit it sexually, or whether men can transmit it to men.


The CDC advises men who had symptoms to either use condoms or avoid sex for six months, and have provided a graphic illustrating condom use for males just in case. If a man has traveled to a Zika-endemic area but hasn’t had symptoms, he should use condoms or abstain for eight weeks.

Back up. Did you say no unprotected sex for six months? Where did they get that number?

In the case with the longest interval, Zika was detected in semen two months after an infection with symptoms. So the CDC tripled that window to be on the safe side.

Do the damn mosquitoes have semen?

Well no, but they do have sperm. In fact, making mosquito sperm defective with radiation and releasing males carrying nonviable sperm is one possible way to control their populations. (This could also help control the spread of Zika. As long as the sexual-transmission route doesn’t gain traction.) Mosquitoes can also get STIs and are capable of transmitting dengue amongst themselves sexually, although there has been no suggestion yet that this is the case with Zika too.

We should make them wear condoms too.


Wait. What if I am female, but I am not planning on getting pregnant?

The CDC suggests you use birth control and avoid all unprotected sex anyway, including “mouth-to-penis oral sex.” This is doubly advised if your partner could have Zika, even if it’s oral sex (though there are no known cases of transmission via that route to date).

Wait, wait. What if I do want to get pregnant?

If you live in one of the 38 countries where there is active Zika transmission, experts recommend you talk to your doctor. And if you have had Zika symptoms or were possibly exposed, they suggest waiting eight weeks before having unprotected sex or attempting to get pregnant. If your partner has had symptoms, you should wait six months.


Wait, wait, wait. What if I’m already pregnant?

Then you are advised to avoid traveling to areas where Zika is spreading. If you have to travel, or if you live in those areas, you should follow mosquito bite-prevention steps. And you should not have any unprotected sex whatsoever with anyone who has traveled to or lives in those areas for the duration of the pregnancy. So while the advice to wear a condom if you’re pregnant may seem strange, it is what the CDC recommends to protect both mother and fetus.


Will an infection affect a future pregnancy, in like five years or something?

No. It is likely that Zika can only affect a fetus if infection happens at the same time the fetus is developing. Once the infection is cleared completely, there should be no future problems.


Is the link to microcephaly a real deal or not?

It seems to be. A team of researchers recently infected human brain stem cells in a Petri dish with Zika virus. They discovered that the virus changes the cell cycle and increases cell death in those cells. Killing off the stem cells that are usually fated to divide extensively and build the cortex of the brain may be the mechanism by which the virus causes microcephaly. They’re not yet sure how to stop this from happening—hence the advice to avoid infection in the first place. [Update, April 14: The CDC has officially confirmed the link between Zika and microcephaly.]


Are there any precedents for a sexually transmitted pathogen infecting fetuses?


Yes. Mother-to-child, or “vertically transmitted” infections include HIV, syphilis, and herpes. They can manifest with slightly different symptoms in babies infected in the womb.

Additionally, rubella infection just prior to or during early pregnancy can cause a syndrome in a baby that includes deafness, blindness, and cognitive development challenges. A vaccine for rubella was developed in 1969. It is currently administered as part of the MMR cocktail vaccination required in the U.S. Because of vaccination, Rubella was declared officially eradicated in the Americas in 2015.

Eradicated. I like the sound of that. Any other hopeful news?

A dozen companies are working on Zika vaccines, some focusing on ones that can be used in pregnant women. The conditions for epidemic spread of the virus in the U.S. may not exist, thanks to Americans’ more indoorsy lifestyles. So far, there haven’t been any cases in the 50 states from mosquito bites—all were in people who traveled or people who had sex with someone who was infected. And Pope Francis said contraception use may be justified in areas where Zika is spreading.

The Pope said people can use condoms? That is kind of a big deal.

It is.

Read more in Slate about the Zika virus.