from Global Health Program

The Zika Virus

Geovane Silva holds his son Gustavo Henrique, who has microcephaly, at the Oswaldo Cruz Hospital in Recife, Brazil. (Ueslei Marcelino/Reuters)


Last updated August 11, 2016

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The Zika virus, a mosquito-borne illness, has been linked to a dramatic rise in birth defects in Brazil and neighboring countries. The World Health Organization (WHO) declared it a Public Health Emergency of International Concern in February 2016, and by mid-2016, sixty countries were reporting active transmission of the virus. Health officials confirmed that the Zika virus is behind a dramatic increase in cases of microcephaly, a condition in which infants are born with unusually small heads and brains that usually results in developmental disabilities. The U.S. Centers for Disease Control and Prevention (CDC) has said pregnant women, or women who may become pregnant, should consider postponing travel to the nearly thirty countries where the Zika virus has been transmitted. Some governments, including those of Colombia, Ecuador, and El Salvador, have advised women against becoming pregnant in the near future.

What is the Zika virus?

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The Zika virus is a mosquito-borne illness carried by Aedes aegypti mosquitoes. Most people who are infected do not become ill, but an estimated 20 percent experience symptoms including rash, fever, joint pain, red eyes, muscle pain, and headaches. The incubation period—the time between exposure to exhibiting symptoms—is unknown, but, according to the CDC, it is likely between a few days and a week. In most cases symptoms are mild and last up to a week.

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The virus was first discovered in 1947 in the Zika forest in central Uganda, but until 2007, there had only been fourteen documented cases in humans. Experts say the disease likely did not spread among humans in Uganda because the Aedes africanus mosquitoes that transmit the virus there are poorly adapted to human environments, and therefore preferred to prey on monkeys. Researchers found evidence of infections elsewhere in Africa, as well as in Asia, but local populations there appear to have developed some resistance to the virus, preventing large-scale outbreaks.

In 2007 officials confirmed forty-nine cases of Zika on the island of Yap, in the Federated States of Micronesia, in the western Pacific. In a 2013–2014 outbreak, nearly four hundred cases were confirmed in French Polynesia, more than five thousand miles southeast of Yap. Researchers say the virus likely arrived in the Americas in 2013.

How the Zika Virus Spread to the Americas

While in most cases symptoms of Zika infection are mild, researchers have found the virus to be responsible for a dramatic rise in birth defects.

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Brazilian health officials have reported more than seven thousand cases of suspected microcephaly since the beginning of 2015, up from 147 cases in 2014. As of July 2016, researchers confirmed just over 1,600 of those cases; thousands more remain under investigation.

Zika has also been associated with Guillain-Barre syndrome (GBS), a rare disorder in which the immune system attacks the nerves, sometimes causing paralysis. Symptoms can last a few weeks, and though most people recover, there have been reports of patients suffering permanent harm, or even death when paralysis reaches the lungs and respirators are not available. In April 2016, an elderly man in Puerto Rico died of complications from GBS, marking the first Zika-related death on U.S. territory.  

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How is it transmitted?

Zika is primarily spread by Aedes mosquitoes. Aedes aegypti has spread most of the cases in the Americas, and its reach in the United States is generally limited to Florida and Hawaii. However, scientists have also detected the virus in Aedes albopictus, known as the Asian tiger mosquito, in Mexico; it has a much wider range in the United States, reaching as far north as New York and Chicago in the summer.

There have been reports of the virus’s sexual transmission, and researchers say it could also be transmitted through blood transfusion. The virus has also been found in saliva and urine, but it is unclear whether it can be spread through those channels. 

Why is it spreading so quickly?

The Western Hemisphere is "immunologically naïve" to the Zika virus, meaning that populations in the Americas have not developed resistance to it because the mosquitoes that carry it are not native to the region. (Aedes aegypti is believed to have arrived on slave ships in the 1600s, and Aedes albopictus in recycled tires shipped from Asia in the 1980s.)  The prevalence of the Aedes aegypti, the most successful vector for Zika, in dense, urban areas in the Americas also contributes to the spread of the virus.  Rapid urbanization and increases in international travel expose more people to more diseases, and changing weather patterns expand the range of mosquitoes.

The Zika outbreak comes as other mosquito-borne illnesses are on the rise.

Extreme weather patterns associated with El Niño—heavy rains in some areas and drought in others—can cause an abundance of standing water, which attracts mosquitoes. (During droughts, people often gather water in open containers.) The Zika outbreak comes as other mosquito-borne illnesses are on the rise: Brazil reported 1.6 million cases of dengue fever in 2015, up from 569,000 the year before. Chikungunya, a virus that causes fevers and joint pains, first detected in the Western Hemisphere in 2013, had by July 2015 infected 1.5 million people in the Americas. Zika and other mosquito-borne illnesses appear to disproportionately affect the urban poor, who are more likely to live in areas with poor sanitation and open water sources, and less likely to have window screens and air conditioning, leaving them exposed to mosquitoes.

Many observers say climate change, increased travel, and urbanization allow the conveyers of such diseases to thrive. "Aedes aegypti, the mosquito that is largely responsible for transmitting pathogens such as Zika and dengue, thrives in the warm, humid, increasingly dense urban centers of Latin America, and climate change has been making these places warmer and wetter," writes the New Yorker’s Carolyn Kormann.

How the Zika Virus Spreads to the Americas

Is there a vaccine for Zika?

No, but several companies and research groups have begun early-stage research to develop a vaccine. A few, including Sanofi SA, a French company that has partnered with the U.S. Army, are expected to begin clinical trials on humans by late 2016. However, U.S. health officials have warned that lack of funding could delay research.

There is no cure for microcephaly or Guillain-Barre, which have been linked to the virus. Speech and occupational therapies can improve cognitive development in children with microcephaly, and plasma exchanges and immunoglobulin therapy can reduce the severity of Guillain-Barre.  The WHO has called for researchers to develop a vaccine and introduce rapid diagnostic testing for the virus. Currently, blood and tissue samples must be sent to advanced laboratories.

Governments and health professionals in many countries in the Americas are urging women who are at risk of contracting the virus to avoid becoming pregnant in the immediate future, something that has revived debate over women’s reproductive rights and access to contraception in the region. (Abortion is illegal in most cases in most Latin American countries.) The CDC recommends Zika testing and possible amniocentesis for pregnant women returning from affected countries with symptoms.

What is the threat to the United States?

CDC officials have said widespread transmission of Zika in the mainland United States is "unlikely," and most of the nearly two thousand cases reported in the continental United States were contracted abroad. U.S. officials identified more than five thousand cases in U.S. territories, including Puerto Rico, American Samoa, and the U.S. Virgin Islands, as of August 2016. The CDC has said hundreds of thousands of people may become infected in Puerto Rico. Pregnant women constituted nearly one thousand of reported cases in the United States and its territories.

By August 2016, health officials in Florida had identified twenty-two locally transmitted cases of Zika, including one pregnant woman. The CDC warned pregnant women to avoid the Wyndwood section of Miami, where mosquitos were believed to be spreading the virus. 

A January 2016 study in the UK-based medical journal, the Lancet, found that around two hundred million people live in areas in the United States that could be affected by Zika in warmer months. CFR Senior Fellow Laurie Garrett warned in early 2016 that Zika could become a permanent fixture in the Western Hemisphere, like the West Nile virus, especially if it takes hold in Culex mosquitoes, which are ubiquitous in the Americas (Brazilian researchers were able to infect a Culex with Zika in a laboratory). 

Researchers point to other mosquito-borne illnesses, such as dengue and Chikungunya, which have not gained traction in the mainland United States, and say the prevalence of air conditioning and window screens in the United States helps to stem the transmission. High-quality sanitation systems, which reduce exposure to standing water, also reduce the risk of transmission.

How are health authorities responding to the outbreak?

The WHO declared the possible link between Zika and neurological disorders a Public Health Emergency of International Concern (PHEIC) on February 1, 2016, and said there was a "strong scientific consensus" of the link in March. The CDC followed suit in April. The PHEIC designation allows the agency to raise funds, coordinate multicountry efforts, and require countries to share health data relevant to the outbreak with international authorities. The organization called for more research on the virus, but did not recommended restrictions on travel to Brazil or other areas with Zika virus transmission. The organization also said pregnant women and women of childbearing age should have access to "necessary information and materials to reduce risk of exposure."

Health officials in Brazil, the epicenter of the outbreak, issued a warning to pregnant women about the possible links between Zika and microcephaly in November 2015, and in February deployed 220,000 troops to distribute information on Zika. El Salvador’s government has warned women not to become pregnant until 2018, and Brazil, Colombia, and Ecuador have advised women to put off becoming pregnant until more is understood about the virus. Pope Francis, during his return from a six-day trip to Cuba and Mexico in February 2016, said the use of contraception may be permissible in regions where Zika was prevalent. Local bishops in the predominantly Roman Catholic region had previously said the Zika outbreak did not justify the use of artificial contraceptives.

Authorities in the region are trying to control the outbreak by fumigating areas with high incidences of infection, removing pools of standing water, and releasing genetically engineered mosquitoes whose offsprings’ short life spans cause overall population decreases.

In February, U.S. President Barack Obama asked Congress for $1.9 billion in emergency funding to combat the virus through mosquito control, vaccine research, and education and health care for low-income pregnant women in the United States. Leaders in the Republican-majority House of Representatives urged health officials to first use reallocated funds of roughly $622 million, and the Senate approved $1.1 billion, figures health officials said would not be enough. Lawmakers had been unable to reach a compromise in order to release funds by mid-July, when Congress was adjourned for a seven-week recess. 

WHO has to prove that it can take charge of dealing with Zika.
Suerie Moon, Professor, Harvard H. Chan School of Public Health

Nearly half a million people are expected to travel to Brazil in August, when Rio de Janeiro hosts the Summer Olympics. Officials say the risk of transmission will decrease during the Southern Hemisphere’s winter months, but some health experts have called for the games to be cancelled. The U.S. Olympic Committee reportedly told sports federations that athletes and staff should not go to Rio if they feared for their health because of the Zika virus. In July, the CDC said the Olympics were unlikely to cause Zika to spread, as the expected travelers to Rio represent only 0.25 percent of global travel to Zika-infected areas.

The Zika outbreak in the Americas comes as the WHO, whose response to the 2014 Ebola outbreak in West Africa was widely criticized, works to improve its emergency response systems. "WHO has to prove that it can take charge of dealing with Zika," writes Suerie Moon, a professor at the Harvard H. Chan School of Public Health and Kennedy School of Government. Moon writes the agency should help advance research on the virus, ensure the affordability of drugs and vaccines, and "communicate to an uneasy global public that Zika can be controlled."

Gabriella Meltzer contributed to this report.



The World Health Organization issued this statement on the Zika virus and clusters of microcephaly cases and neurological disorders.

CFR's Laurie Garrett warns that the Zika virus could become endemic to the Western Hemisphere in this Foreign Policy article.

The New Yorker's Carolyn Kormann looks at the spread of Zika to the Western Hemisphere in recent years.

In this New York Times op-ed, Brazilian rights activist Debora Deniz argues that the Zika epidemic mirrors social inequalities in her country.

Laurie Garrett and Brazilian public health official Cláudio Maierovitch Pessanha Henriques discuss the outbreak in this CFR Conference Call.

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