Medical Examiner

Zika Facts to Calm and Terrify You

The mosquito-borne disease, explained.

The Aedes aegypti mosquito, one of two species that transmits Zika.

Centers for Disease Control and Prevention/Wikimedia Commons

This article has been updated with new information since it was originally published. 

So, what the hell is Zika?

It’s a virus. Specifically, it’s an arthropod-borne, or arbovirus. It is carried by mosquitoes and transmitted by their bites.

Mosquitoes are the worst.

Well, only two species of mosquito carry Zika—Aedes aegypti and Aedes albopictus.

A. aegypti used to be called the “yellow-fever mosquito.” It was originally from Africa but has now spread to tropical and subtropical climates all around the globe. A. albopictus, aka the Asian “tiger mosquito,” is a similar species and is thought to have traveled from Asia in barges full of used tires. Then they started hitch-hiking. By 1997 they’d colonized warm counties along the U.S. interstate system. Back in the day they were not considered a vector for human diseases. Now they’re known to carry aegypti-friendly viruses such as dengue and chikungunya.


Chikungunya? Is that some kind of fusion taco?

No. Like Zika, chikungunya is a virus that causes fever, fatigue, joint pain, and a rash. The name is a Bantu dialect for, like, all bent up, because of the contorted posture caused by joint pain. U.S. mosquitoes have already begun transmitting that one in Florida. Plus Lindsay Lohan had it after getting a mosquito bite in French Polynesia.

So what are the symptoms of Zika?

Most people (up to 80 percent) who get infected don’t have any symptoms at all. Otherwise, the initial signs are banal, chikungunya-esque ailments including mild fever, fatigue, headache, joint pain, and bloodshot eyes. Those are actually symptoms of a dengue infection as well, so lab tests are needed to discriminate between Zika, dengue, and chikungunya (or to detect co-infections). Something called a maculopapular rash—red circles covered with little bumps—might also be a telltale symptom of Zika. Symptoms last about a week, and by all accounts fatalities are very rare.


Then why is everyone so worked up about it?

Mostly because of the explosion in infection rates and the invasive penetration into new territories.

But the most disturbing new development is the growing incidence of babies born with microcephaly in Brazil. Rates of that condition, usually defined as having a head circumference of more than two standard deviations below the mean for sex and gestational age at birth, have spiked in recent months. Where there were fewer than 200 cases in each of the previous two years, now there have been more than 4,000 since October.


A subset of these babies had mothers who lived or traveled in areas where there was a known Zika outbreak. In one small sample, about three-quarters of the moms reported having had a rash during the first or second trimesters.


That said, there is still no proof that the virus causes either microcephaly or Guillain-Barré syndrome, a rare form of paralysis that has also been linked to the disease. [Update, April 14: The CDC has officially confirmed a link between Zika and microcephaly.] Still, the correlation in Brazil has gotten the global public health community pretty riled. They are erring on the side of caution. The U.S. Centers for Disease Control and Prevention now recommends that pregnant women returning from places with ongoing infection who also have two or more Zika symptoms should get tested.

Wait, are people bringing Zika to the U.S.?

Yes. There have been isolated travel-related cases so far in Florida, Arkansas, California, Virginia, Minnesota, Illinois, New Jersey, Texas, and New York. There was one case of microcephaly in Hawaii linked to travel during pregnancy.


What should we do now? Panic? Is the answer panic?

No. A mosquito has to bite an infected person to pass on the virus, and so far there has been no local transmission in the U.S. Also, it would have to be an A. aegypti or A. albopictus mosquito, and those are still limited to warmer climates. But if you live in some of the current outbreak countries, and you are female, you are advised not to get pregnant for now. Jamaican authorities say wait a year. El Salvador advises avoiding pregnancy until 2018. Birth control options, as well as access to abortions, are limited in parts of the Americas.


So, basically countries are telling people not to have sex for two years?


It would seem so.

Where was the first Zika epidemic, anyway?



Yup. Yap used to have amazing tattoos and stone money. Now they also have new infectious diseases. Zika virus was first isolated in 1947, from a rhesus monkey’s blood in the Zika forest of Uganda. Then, 60 years later, an outbreak on the Yap islands in Micronesia provided the first documented case of more than 10 people infected with Zika at once.


I just ate something disagreeable, and I would like to vomit. Please tell me the most emetic fact about Zika that you know.

In 2008, two scientists were collecting mosquitoes in Senegal to bring back (dead) and study in their Colorado lab. Both got sick within a few days after returning to the U.S. One of them had the usual Zika symptoms, along with “red-brown fluid in his ejaculate,” as noted by his wife, who was also a nurse. The wife got sick a few days later, likely due to sexual transmission of the virus.

I’m made of steel and revel in infectious disease porn. Got one last factoid for me?

One model of Zika spread includes places that have both the mosquito vectors and airports. Researchers mapped final destinations of about 9.9 million travelers who departed Brazil between September 2014 and August. Nearly 3 million came home to the U.S.

Oh, wait. Aren’t the Olympics in Brazil this summer?

Yes, yes they are.

Read more in Slate about the Zika virus.