This post was updated on November 28, 2016.

Scientists have known about the Zika virus for seven decades, but human infection has only been a serious public-health concern in recent years—especially amid outbreaks that have taught researchers more about emerging, mutated strains of the virus.

There is still much that we don’t understand about Zika, but we do know it’s a catastrophic threat to global public health—even despite the fact that the majority of those who are infected never experience telltale symptoms such as fever, rash, joint pain, red eyes, and muscle aches. This makes it particularly hard to test for—Zika only shows up in a person’s bloodstream about one week after infection, so there’s a fairly small window of time in which a Zika diagnosis can be confirmed through a blood or urine sample.

Zika is of particular concern to pregnant women, whose fetuses can suffer grave outcomes when a woman contracts the virus. Yet the virus can be serious—and in rare cases, deadly—for children and adults, too.


What We Know

Zika primarily spreads through infected mosquitoes. The main carrier of the virus is the Aedes aegypti mosquito. (The more geographically disperse Aedes albopictus can also, more rarely, spread the disease.)

You can also get Zika through sex, including vaginal sex, anal sex, oral sex, and the sharing of sex toys, according to the CDC:

Zika can be passed through sex, even if the person does not have symptoms at the time. It can be passed from a person with Zika before their symptoms start, while they have symptoms, and after their symptoms end. It may also be passed by a person who has been infected with the virus but never develops symptoms. Studies are underway to find out how long Zika stays in the semen and vaginal fluids of people who have Zika, and how long it can be passed to sex partners. Current research shows that Zika can remain in semen longer than in other body fluids, including vaginal fluids, urine, and blood.

In one rare case, the virus appears to have been transmitted via casual contact—but this is not typical.

The Zika virus can be passed from a pregnant woman to her fetus, and an infection during pregnancy can cause birth defects. Microcephaly, in which a baby is born with an abnormally small head and underdeveloped brain, is one danger. And some children who don’t show signs of microcephaly at birth can develop it as they age.

Congenital Zika syndrome is the more general term for a pattern of birth defects that could result from the virus—including severe microcephaly, decreased brain tissue, damage to the eye, clubfoot, and muscle development problems that lead to restricted movement soon after a baby is born. Scientists believe that the first trimester is the most dangerous, but a Zika infection in the mother could harm the fetus at any point during pregnancy.

In August 2016, the CDC issued updated recommendations on how people planning to get pregnant should protect themselves from Zika. For those who have either traveled to a Zika-infected country or had unprotected sex with someone who has, women should wait at least eight weeks since last possible exposure before trying to get pregnant and men should wait at least six months.


What We Don’t Know

Scientists know that Zika can be harmful to the developing brain, but it’s unknown when the threat of serious damage passes. It’s not clear, for instance, how likely it is that the virus might cause irreparable harm to a toddler’s brain.

It’s also unknown exactly how the risks associated with Zika change over the course of a woman’s pregnancy, so the CDC can’t say whether there’s ever a safe time during pregnancy to travel to an area with Zika.

If a woman is infected with Zika while she is pregnant, scientists don’t know how to determine whether the baby will have birth defects—or even developmental delays related to Zika that aren’t physically detectable. That’s why Zika has been described as a “delayed epidemic.” It’s likely to be several years or more before the true scope and gravity of the outbreak is understood.

It’s also possible that Zika’s effects could be compounded by other mosquito-borne viruses—dengue and chikungunya, specifically. The three viruses were spreading at once through Brazil in 2015, and some people got sick with more than one at the same time. Researchers have found that mosquitoes can carry Zika and chikungunya at the same time, and could potentially infect a human with both at once. But it’s unknown as of now whether co-infection with two or more of these viruses may play a role in the neurological complications associated with Zika.

There is no vaccine to prevent Zika, and no medicine to treat the virus. In the past year, however, scientists have made major strides toward vaccination and prevention. Several vaccines are in the works, including two that have entered clinical trials in humans at the National Institutes of Health. Scientists are also exploring the efficacy of using genetically modified mosquitoes to stop the spread of the virus. One method involves releasing lab-sterilized mosquitoes into the wild, and the other involves using a common bacteria against wild Zika-carrying mosquitoes.

In the meantime, the best defense against Zika at the government level is mosquito control efforts. Individuals should review travel advisories and use bug spray in affected areas. Contraception is also key to preventing sexually transmitted Zika.


Where in the World Is Zika?

As of October 2016, the CDC has issued travel warnings for Zika in most countries throughout the Americas, as well as in the Caribbean, Asia, Southeast Asia, and the Pacific Islands.

CDC

The risk in countries where Zika is endemic is not zero, but it is considered much lower than the risks associated with the virus in countries where outbreaks are active. Zika is endemic in parts of Africa (Angola, Benin, Burkina-Faso, Cameroon, Central African Republic, Côte d’Ivoire, Egypt, Ethiopia, Gabon, Guinea-Bissau, Kenya, Liberia, Mali, Mozambique, Niger, Nigeria, Senegal, Sierra Leone, Somalia, Tanzania, Togo, Uganda, Zambia); Asia (Bangladesh, India, Pakistan), and the Pacific Islands (Easter Island, Vanuatu).


Where Did Zika Come From?

Zika was first discovered in monkeys in Uganda in 1947. The first human cases of the virus were detected in the region in 1952. Up until the 1980s, humans across Africa and Asia contracted the virus, but those who did typically only suffered mild illness. It wasn’t until 2007 that a large outbreak was recorded, this time on Yap island in Micronesia. In another massive outbreak, this one in French Polynesia in 2013, researchers found a link between Zika and the neurological disorder Guillain-Barré syndrome. Then, in mid-2015, Brazil began to see reports of locally-transmitted Zika. Some scientists believe that Zika got to Rio de Janeiro from French Polynesia during the Va’a World Sprint Championship canoe race in August 2014.

Since then, a dangerous strain of the virus has continued to spread. As of November 2016, the Centers for Disease Control and Prevention had recorded 4,128 cases of Zika in the United States—and 30,178 more cases in U.S. territories—including more than 3,000 cases of Zika confirmed in pregnant women in those regions.

In Puerto Rico, especially, Zika is spreading rapidly. “If current trends continue, at least 1 in 4 people, including women who become pregnant, may become infected with Zika” there, according to the CDC.

Meanwhile, since the new Zika strain started spreading in Southeast Asia in September 2016, the CDC has expanded its travel warning for pregnant women, cautioning them to avoid travel to 11 countries in Southeast Asia: Brunei, Burma (Myanmar), Cambodia, Indonesia, Laos, Malaysia, Maldives, Philippines, Thailand, East Timor, and Vietnam.


Recommended reading:

Tiny Vampires

Tracing Zika Back to Patient Zero

Pesticides Aren’t the Best Way to Fight Zika-Carrying Mosquitoes

The First Documented Case of Zika Spread by Physical Contact

Mosquitoes Can Pass Zika to Their Offspring

A Surprising Link Between Zika and Dengue

Zika Seems to Thrive in the Vagina

Can Women Spread Zika Sexually, Too?

The Potential Zika Threat to Adult Brain Cells

The First Known Zika-Related Infant Death in the U.S.

The Mystery of Zika’s Path to the Placenta

Toward an Understanding of Zika's Neurological Dangers

Zika Linked to Ever More Neurological Conditions

The Sneak-Attack Mosquito

There's Probably Way More Zika in the United States Than Has Been Counted

Pregnant in Miami With Zika on the Loose

Zika's Growing Presence in Florida

Zika in Puerto Rico

The Link Between Zika and Climate Change

A Zika Catastrophe Could Rival Hurricane Katrina

The Other Zika Mosquito

Zika Is a Delayed Epidemic

The Danger of a Third-Trimester Zika Infection

What Zika Researchers Can Learn From the Rubella Outbreak of 1964

When in Pregnancy Is Zika Most Dangerous?

Will Winter Kill Zika?

How Does Zika Affect the Toddler Brain?

If a Pregnant Woman Gets Zika, What's the Risk of Microcephaly for the Baby?

Genetically Modified Mosquitoes: What Could Possibly Go Wrong?

Zika Is the ‘Most Difficult’ Emergency Health Response Ever, CDC Official Says

The Answer to Zika May Be More Mosquitos

Three Kinds of Vaccines Protect Monkeys from Zika

How to Beat Dengue and Zika: Add a Microbe to Mosquitoes

Parents Are Being Sold Zika Protection That Doesn't Work

Keeping Mosquitoes at Bay in the Age of Zika

A Threat Bigger Than Zika

The Importance of Contraception to the Zika Fight