Updated by Julia Belluz and Javier Zarracina on February 2, 2016, 9:30 a.m. ET
The Zika virus was first discovered in the 1940s, though most people had never heard of it until this year. That’s because for decades, Zika outbreaks were sporadic and tiny, and the disease seemed to do little harm. That changed in 2015. A massive outbreak in Brazil — affecting more than 1 million people — has changed our view of the mosquito-borne virus. Scientists are learning that Zika may actually be a lot more dangerous than anyone thought, potentially damaging the brains of fetuses and causing incurable and lifelong health and cognitive problems. In light of this evidence, the World Health Organization declared a public health emergency on February 1.
Meanwhile, the virus has been spreading throughout the Western Hemisphere at a rapid rate, carried by a type of mosquito that feeds on and thrives alongside humans. More than 20 countries are currently battling outbreaks, and Zika is expected to reach nearly every corner of the Americas this year (save for Canada and Chile, which aren’t home to the mosquito in question). Odds are you’ll be hearing a lot more about Zika in the coming weeks and months.
1) I’d never heard of Zika. Is it a new virus?
Zika is actually an old virus — it’s only recently that health experts have been seriously worried. It was first discovered in 1947 when it isolated from monkeys in the Zika forest in Uganda. And for decades thereafter, it barely bothered humans.
Soon cases popped up in other Pacific Islands, including a large outbreak in 2013-’14 in French Polynesia (388 cases). By May 2015, health officials had detected the virus in Brazil — possibly arriving with a traveler to the World Cup. Within a year, more than a million people in Brazil had been affected, as mosquitoes carried it from person to person as they do diseases like malaria and yellow fever.
Zika has since spread to more than 20 countries — mostly concentrated in Central and South America and the Caribbean — and it’s expected to go much further.
2) What happens if you get Zika?
It really depends.
One of the things that makes Zika very difficult to track is the fact that in the vast majority of cases, it 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.
Meanwhile, 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.
But that’s not the whole story. In rare occasions, Zika seems to cause really serious problems.
In both the Brazil and French Polynesia outbreaks, researchers noted that some people infected with the virus were later diagnosed with Guillain-Barré, a rare and sometimes deadly neurological condition in which people’s immune systems damage their nerve cells, leading to muscle weakness and even paralysis. The symptoms can last weeks, months, or even years.
Even more worryingly, there’s evidence that Zika is linked to a terrible birth defect called microcephaly, which is characterized by a shrunken head and incomplete brain development. (You can read more about microcephaly in our explainer here.)
Since Zika arrived in Brazil in 2015, more than 4,000 cases of microcephaly have been reported — a twentyfold increase from previous years.
The timing appears to be no coincidence. The virus been found in the amniotic fluid of pregnant women carrying babies with the birth defect, and even in the brains of babies with microcephaly who died within 24 hours of being born.
What’s more, reanalyses of the data from previous outbreaks, such as one in the French Polynesian islands in 2013-’14, revealed a rise in birth defects following the arrival of Zika.
But this is not a closed case. Researchers are still working to confirm the link — could it be something else that’s causing the microcephaly? — and there are major questions about the frequency of the condition. As the health ministry in Brazil works to confirm the suspected cases, they’re discarding more than they’re confirming. (As of January 29, of the 4,180 reported cases, 270 have been confirmed and 462 thrown out. Only six of the confirmed cases have been linked to Zika so far.)
Still, Zika wouldn’t be the first virus to cause microcephaly. (Rubella famously caused an epidemic of birth defects before the advent of the vaccine.) And even if this turns out to be a very rare complication of Zika, the imperfect evidence alone was enough to prompt the World Health Organization to issue a rare global public health emergency declaration.
3) What are pregnant women supposed to do about Zika?
The difficulty with Zika is that many people who get the virus are never diagnosed — yet it might still potentially damage fetuses.
This fact has put pregnant women and would-be moms in an awful bind. In some places in Latin America, women are being told to avoid having children for months or even years because of Zika. This isn’t easy: In many of these countries, family planning is nonexistent, condoms are out of reach for some, and abortion is illegal.
Meanwhile, American women who are pregnant have been advised to stay out of countries where the Zika virus is circulating. The Centers for Disease Control and Prevention also issued guidance on how to care for pregnant women during a Zika outbreak, and whether 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 virus 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’re traveling to a place with Zika and worried, read our travel explainer.)
4) How exactly is Zika spread?
Zika is mainly carried by a specific type of mosquito called Aedes aegypti, which spreads the disease through bites.
There’s some experimental evidence suggesting 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.
What makes the Aedes aegypti a unique threat is that it is remarkably effective at carrying viruses — it’s also the primary vector of the yellow fever, dengue, and chikungunya viruses.
Aedes mosquitoes 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. (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.) This is different from other types of mosquitoes, which prefer larger bodies of water.
Mosquitoes may not be the only way to spread Zika. There have been two studies in the medical literature that suggest Zika can be sexually transmitted. In one case, a man who traveled to Senegal and contracted Zika gave it to his wife through intercourse after he returned home. In another, Zika was isolated from semen.
Researchers aren’t sure how long Zika can remain in semen. And it’s not clear how common sexual transmission actually is. Right now the evidence is pretty limited. For this reason, the CDC hasn’t issued any warning yet about the risk.
6) Is there any vaccine or cure?
No. Until recently, Zika didn’t seem to pose much of a threat to human health. So research on the virus has been extremely limited. This outbreak has spurred funding and attention on Zika science, and the WHO has called for researchers to develop a vaccine for Zika as well as better diagnostic testing to detect the virus. This, however, will likely take years.
7) How can we stop more Zika outbreaks?
Since Zika is carried by mosquitoes and there’s no vaccine, the best way to prevent outbreaks is to prevent mosquitoes that carry the virus from biting people.
Health officials can treat certain bodies of water with larvicide to eliminate mosquito breeding sites. They can also spray adult mosquitoes with fumigation to kill them off.
Since Aedes mosquitoes prefer small bodies of water around people’s homes, there are also important and effective measures that individuals can take to limit mosquitoes’ habitats, says Janet McAllister, a researcher in the division of vector-borne diseases at CDC.
“Just generally keeping your yard clean and in shape, getting rid of debris or things stored outside that can hold water, or adjusting them so that they no longer hold water,” McAllister says. “It’s also important to maintain the vegetation in your yard — to keep the grass mowed and bushes trimmed, with good air flow.” Campaigns to clean up people’s yards and work sites, and instituting tire pickup programs (old tires are a popular breeding place).
Inside the home, the CDC recommends making sure you have screens on windows and doors that aren’t punctured in any way. Wearing protective clothing, using insect repellents, and keeping your house cool with air conditioning also helps.
Finally, one controversial method of controlling mosquitoes would be to use genetically modified Aedes aegypti mosquitoes that can essentially help sterilize the population. But it’s not entirely clear that this will be a panacea, and if human’s past battles with mosquitoes are any indication, killing them off won’t be easy.
8) Why did the outbreak in Latin America spiral out of control so quickly?
There are a number of reasons that seem probable, and others that will come into focus as we learn more about the outbreak.
First, because Zika didn’t seem to pose much risk to humans, this virus wasn’t exactly on the world’s watch list. It wasn’t even a reportable disease in the US, meaning doctors who found cases didn’t have to alert the CDC as they would with other serious diseases. In other words, health authorities weren’t anticipating an outbreak of Zika, and therefore were caught unprepared.
Second, Zika had never been recorded in the Western Hemisphere until it hit Easter Island off Chile in 2014. That means people living in the Americas are susceptible to the virus, since nobody has built up the antibodies from previous infections to fight it off.
Third, Aedes mosquitoes live all over Latin America. Couple that with the fact that many people live in communities that are perfectly hospitable to these insects: There’s little air conditioning and window screens (to keep mosquitoes out) as well as poor sanitation and a lack of access to clean water (so people store water around their homes). In this environment, it makes sense that Zika has “spread explosively.”
Not to mention that the spread of Zika is actually part of an unnerving trend: Several mosquito-borne tropical illnesses (dengue, chikungunya) have lately been spreading into regions of the world that have never experienced them. Researchers don’t fully understand why this is happening, but they suspect the increasing popularity of global travel and the warming of the climate have something to with the change.
9) Will Zika spread in the United States?
So far, local transmission of Zika virus has not been identified in the continental United States, but there have been cases in Puerto Rico and the Virgin Islands. And travelers returning to a number of states have tested positive for the virus.
Officials are predicting that Zika is likely to follow the same pattern as dengue fever in the United States — reaching Puerto Rico first, followed by outbreaks in Florida and other Gulf Coast states, and maybe Hawaii — all regions where the Aedes aegypti mosquitoes live. Most likely, an outbreak would start like this: A traveler carrying the virus would return to a place with the Aedes aegypti, and the mosquito would pick up the virus and infect others through bites.
But there’s at least some good news: Outbreaks here are expected to be small and local.
“Better housing construction, regular use of air conditioning, use of window screens and door screens and state and local mosquito control efforts helped to eliminate [mosquito-borne infections like malaria] from the mainland,” said Lyle Petersen, director of the CDC’s division of vector-borne diseases, in a recent statement. These conditions will likely protect Americans, but they aren’t present in many developing countries, which suggests Zika will be with us for a long time to come.