Read more of our articles on the Zika virus here.

Researchers suspect that a poorly understood virus is linked to an alarming number of babies born with microcephaly in South America. The Zika virus is not new—there have been outbreaks in Africa, Southeast Asia, and the Pacific Islands for decades—but the number of cases is quickly growing in a new part of the world. On Monday, the World Health Organization (WHO) declared the outbreak to be an international public health emergency. Spread by a mosquito that thrives in tropical climates, the virus is hard to prevent; so hard, in fact, that some governments are asking women not to get pregnant until they have the outbreak under control.

The Zika virus is spread by Aedes mosquitoes, which are also known to spread dengue, chikungunya, and yellow fever. Most people infected with the Zika virus won’t ever know they have it. According to the Centers for Disease Control and Prevention (CDC), only about one in five people infected report symptoms, and those are usually quite mild. The disease often begins with a fever and rash, and can also cause joint pain and conjunctivitis, also known as pink eye. The symptoms last from between two days and a week.

What makes this virus scary, however, is the effect it is suspected to have on fetuses when pregnant women become infected. The exact relationship between microcephaly in babies and the Zika virus is not yet understood, but evidence suggests that the current outbreak of the virus in Latin America is related to 4,000 babies born with the condition in Brazil since May 2015.

Microcephaly is an uncommon condition in which a baby’s head is much smaller than expected. It can occur because a baby’s brain has not developed properly during pregnancy or has stopped growing after birth. In the United States, the condition occurs in approximately 2 in 10,000 live births. Babies born with microcephaly often suffer from other health issues such as seizures, feeding problems, hearing loss, vision problems, developmental delays, and intellectual disabilities, all of which can vary in severity.

The exact causes of microcephaly in most babies are not known: Though it can be genetic, it can also be the result of prenatal exposure to toxic chemicals, drugs, alcohol, or certain infections, such as rubella, toxoplasmosis, and cytomegalovirus. The sheer number of babies born with microcephaly in Brazil during the current outbreak of Zika is a strong indicator that the virus is somehow responsible, although the link has not been as evident in other countries where it has spread.

According to the CDC, the Zika infection usually lasts in a patient’s blood for about a week; it does not pose a risk for future pregnancies.

As of now, there is no way to prevent Zika virus other than to avoid the mosquitoes that cause it; a vaccine study in humans may begin this year. For this reason, the CDC is recommending that pregnant individuals consider postponing travel to the regions affected by the virus, and that those planning to become pregnant talk to their doctors before they travel to these areas.

For people who live there already, however, the guidance is very different. Many countries—including Brazil and Colombia—have advised women not to get pregnant until the crisis has passed. Government officials in El Salvador have taken it even further, and asked women to postpone pregnancy until 2018. After thousands of cases of the Zika virus were detected in the El Salvador in the first weeks of the year, Deputy Health Minister Eduardo Espinoza announced, “We are recommending that women of childbearing age take the precaution of planning their pregnancies and try to avoid pregnancy this year and next.”

The WHO says it would not advise suspending pregnancies for two years, and public health experts say this is the first time that they’ve ever heard such advice coming from a government body. David Bloom, a professor at the Harvard School of Public Health, told the New York Times, “I can tell you that I’ve never read, heard, or encountered a public request like that.”

Dr. Howard Markel, a professor of the history of medicine at the University of Michigan, agreed and explained to the Times that it reminded him of the early days of the HIV epidemic, before there was any way to prevent transmission from mother to child. He said, “There was some sotto voce debate about whether it was morally ethical for a doctor to advise a woman not to get pregnant because of the risk to her child. … But no one said, ‘It’s verboten, don’t do it.’”

The advice to postpone pregnancy is particularly complicated in countries in which contraception is not widely available and abortion is illegal.

Despite the strong influence of religions in the region that resist many forms of contraception, the majority of married women in Latin America use a modern method of birth control. In Brazil, more than three-quarters percent of married women ages 15 through 49 use a modern method of contraception; in Colombia, 73 percent of these women do; and in El Salvador, 68 percent. Still, the WHO believes that there is an unmet need for contraception in this region, especially among adolescents, poorer populations, and unmarried women. It estimates that about 10 percent of women in the region who need contraception do not have access to it.

As Kathy Bougher wrote for Rewire about contraception availability in El Salvador:

[In a] study-in-progress carried out by the feminist organization Organización de Mujeres Salvadoreñas por la Paz (Organization of Salvadoran Women for Peace, known as ORMUSA), which shared a preliminary draft with Rewire, early findings based on interviews indicate that although local health centers might prescribe contraceptives, centers can go for months at a time without actually having any in stock. Young women say they routinely encounter humiliating treatment or have their requests to purchase contraception denied at public clinics and private pharmacies.

In addition, the study reports, although the country’s policies direct that there be specialized services and personnel trained to serve adolescents and young adults, in reality those services rarely exist. Gang violence and territoriality also impact clients’ ability to physically access clinics, and the reporting of rapes for fear of retribution.

Paula Avila-Guillen of the Center for Reproductive Rights told the Huffington Post, “These recommendations are really empty words. They aren’t going hand in hand with policies to make contraception and emergency contraception available, especially in El Salvador where those things are very inaccessible.”

None of these countries have, thus far, announced plans to make birth control more available. And religious leaders, especially those who have historically been against contraception, have yet to weigh in. El Salvador’s auxiliary bishop, Gregorio Rosa Chávez, suggested last weekend that the bishops were discussing this issue, saying in an interview that he expected Church leaders to take the situation very seriously.

For those already pregnant, there is no cure for microcephaly, even if it is detected in the womb. In Brazil, abortion is permitted only to save a woman’s life; in Colombia, abortions are legal in cases of fetal anomaly but often very difficult to obtain because of physicians’ reluctance to perform them. El Salvador has such strict laws against abortion that women who are suspected of attempting abortion, possibly because they have suffered a miscarriage or stillbirth, have been jailed for homicide. Women in these countries who find out in the second or third trimester that their fetus has microcephaly may or may not want to terminate the pregnancy; however, they have no choice but to expect to carry to term or seek an illegal abortion. In 2008, there were 32 abortions per 1,000 women in Latin America, 95 percent of which were considered unsafe.

The requests for women to avoid pregnancy is not the only attempt by governments of these nations to prevent Zika’s spread. Brazil is sending 220,000 members of its armed forces into the most heavily hit areas, according to the Guardian, to try to eradicate the Aedes mosquitoes. The soldiers will go house-to-house to distribute leaflets and make suggestions about what people can do to limit the mosquito population, such as emptying all sources of standing water around their homes. They will also provide advice for preventing mosquito bites, such as covering as much as the body as possible with light-colored clothing, closing doors and windows, sleeping under mosquito netting, and using repellent, which is becoming hard to find in the country.

Despite this effort, however, Brazil’s health minister is not optimistic. He noted that his country had already failed in its efforts to eradicate this insect when it was responsible for outbreaks of dengue, chikungunya, and yellow fever. He told reporters, “The mosquito has been here in Brazil for three decades, and we are badly losing the battle against the mosquito.”

El Salvador’s vice minister of health also promises that asking women to put off pregnancy is not the country’s primary strategy—its officials, too, are trying to get rid of standing water and have asked religious leaders to get congregations to clean up trash in the streets that can also be breeding grounds for insects. But, he says the secondary strategy of pregnancy prevention is necessary because “of the fact that these mosquitoes exist and transmit this disease.”

Many of the issues that these countries are facing are not problems in much of the United States because of geography and resources. The mosquitoes likely to carry the virus are limited to the warmer, more southern parts of the country. And because of the spread of West Nile virus and other mosquito-borne illnesses, many municipalities are already careful to eliminate pools of standing water; some even spray insecticides. Finally, although the Zika virus is still fairly little-known, it does not appear to be carried by birds, which is one of the things that makes the spread of West Nile so hard to prevent.

Though there have been cases of the Zika virus reported in the contiguous United States, thus far, all seem to have been contracted in another country. Dr. Beth Bell, director of the CDC’s National Center for Emerging and Zoonotic Infectious Disease, told NPR that she doesn’t expect to see a full-fledged outbreak here.

The CDC and scientists around the world are carefully studying the current outbreaks to learn more about the virus including confirming that it is, indeed, the cause of microcephaly and determining when in pregnancy infection is most risky. Some reports have also suggested that like many other viruses, Zika might be sexually transmitted through the semen of men who have had the illness. This has not been confirmed, and even if it were true, it would undoubtedly account for far fewer cases than those transmitted by mosquitoes. Unfortunately, some research is made harder by the fact that the virus does not infect most lab animals such as mice and rats.

While scientists gather information, women who are pregnant or planning to become pregnant are left to decide how much they are willing to do to prevent the disease. Some can just avoid the areas of outbreak, but others who do not have the luck of geography are left to decide if they are willing—or able—to avoid pregnancy or childbirth altogether.