Zika Virus Infection

Zika (ZEE-ka) virus is a microbe transmitted to humans by mosquitoes. Although most people have mild or no symptoms, Zika infection of a pregnant woman can cause serious birth defects in the fetus.

Maria's Story

Maria and her husband vacationed in Mexico when she was three months pregnant. A week after they got home, Maria had a low fever and joint pain for a few days. Soon news stories appeared explaining how infection with Zika virus, which is found in the area where Maria had vacationed, caused minor symptoms like the ones she had but could cause serious birth defects in unborn children. Maria visited her obstetrician and had a blood test to determine if she had been infected with Zika. Fortunately she had not been infected and months later gave birth to a healthy baby.

What Is Zika Virus Infection?

Zika is a virus belonging to the Flavivirus (FLAY-vih-vy-rus) family. Other viruses in this family include dengue (DENG-gay) virus, West Nile virus, yellow fever virus, and chikungunya (chee-ken-GOON-yuh) virus. The virus is named for the Zika forest in Uganda, Africa, where it was discovered in 1947. As of March 2016, the virus was found in warm, damp parts of Africa, Asia, the Pacific Islands, and some countries in the Caribbean, Central America, and South America.

In May 2015 the Pan American Health Organization issued a warning about an outbreak of Zika in Brazil. In February 2016 the World Health Organization (WHO) declared Zika virus a public health emergency with the potential to spread to many countries. As of mid-March 2016, the virus had been found in 21 Caribbean, South American, and Central American countries. The spread of Zika, its relationship to birth defects, and techniques for diagnosis and prevention were changing rapidly as of March 2016. People are urged to consult the website of the U.S. Centers for Disease Control and Prevention (CDC) for the most recent information on Zika virus infection.

Most people who are infected with Zika have only mild symptoms that last a few days to a week. However, after the 2015 outbreak of Zika in Brazil, doctors noticed an alarming increase in the number of children with microcephaly born to women who had been infected with the Zika virus while pregnant. Microcephaly is a condition in which a baby's head is much smaller than normal. This can restrict brain development and lead to physical and intellectual disabilities that range from mild to severe.

* . In addition, some babies of mothers who were infected late in pregnancy were stillborn (born dead). Much remains unknown about the effects of the timing of the infection on fetal birth defects, although it appears that the virus targets fetal nervous system tissue.

How Common Is Zika Virus Infection?

The frequency of Zika virus infection is difficult to measure because the virus normally causes only mild symptoms that do not require a visit to the doctor. In addition, Zika is found in many places where health care is limited and record keeping is poor. It does appear that Zika infection most often occurs in waves or outbreaks followed by periods when there is only a low level of infection.

As of March 2016, all cases of Zika infection in the United States had been in people who had recently traveled in areas where the infection was active or, in rare cases, had been acquired through sexual intercourse with an infected man. However, most of the southeast United States has a climate suitable for the Aedes mosquito (also called the yellow fever mosquito). These mosquitoes are poor fliers, capable of traveling on the average of 400 meters or 437 yards, but can accidentally be introduced through the importation of infected shipments or water contaminated with mosquito eggs.

Is Zika Virus Contagious?

Zika is transmitted by the bite of two species of Aedes (a-E-deez) mosquitoes. These mosquitoes also transmit dengue and chikungunya virus, which are found in the same areas as the Zika virus. The mosquitoes are aggressive daytime biters but will also bite at night. Their favorite targets are humans. Being bitten by an infected mosquito is the greatest risk factor for becoming infected. The virus is not transmitted by casual contact with infected individuals.

Zika virus can also be transmitted by sexual intercourse. Zika has been found to last up to two weeks in the semen of infected men. It is not clear whether the virus can be transmitted by oral sex or through vaginal secretions or saliva.

Transmission of Zika virus through blood transfusion has also been reported. The virus remains for about one week in the blood of individuals who have been infected. As of March 2016, tests were under development to protect the blood supply, and people who had been to areas where there was known Zika infection were discouraged from giving blood. A risk exists for intravenous drug users because the virus can be transmitted through blood-to-blood exposure through shared needles.

Zika virus can be passed from mother to newborn during delivery. The virus has been found in breast milk, but it is unclear whether this causes infection in the child. As of March 2016, breastfeeding by infected mothers was still recommended.

There is no concrete evidence that domestic animals or pets can be infected or pass on the virus to humans.

How Do People Know They Have Zika Virus?

One of the biggest problems with Zika is that 80 percent of people who are infected show no signs or symptoms and do not know that they are infected. The other 20 percent of infected people usually have mild symptoms, the most common of which are a low fever, rash, joint pain, and red eyes. These symptoms last several days to one week but rarely make people sick enough to visit a doctor.

How Do Doctors Diagnose and Treat Zika Virus?

Diagnosis of Zika is complicated by the fact that most people have few or no symptoms. In addition, some of the symptoms are similar to (but milder than) symptoms of dengue fever and chikungunya, which are found in the same regions as the Zika virus. Diagnosis is made based on travel history and symptoms. The diagnosis can be confirmed by growing the virus in the laboratory from a sample of an individual's blood. The blood can also be tested for antibodies * to the virus, but this test can give false positive readings because the testing material also reacts with antibodies to the dengue and chikungunya viruses. There was, as of March 2016, no way to test semen for the presence of Zika virus.

The only treatment for Zika infection is rest, drinking plenty of fluids, and taking an over-the-counter pain reliever for discomfort and fever. Acetaminophen (uh-see-teh-MIH-noh-fen), also known by the brand names Tylenol and Panadol, and as paracetamol in the United Kingdom, is the recommended pain reliever. Nonsteroidal anti-inflammatory pain relievers such as aspirin, ibuprofen (Advil), and naproxen (Aleve) can cause bleeding in some infected individuals.

Can Zika Virus Be Prevented?

No vaccine exists to treat Zika virus infection. Behavioral prevention consists of:

See also Birth Defects: Overview • Global Health Issues: Overview • Guillain-Barré Syndrome • Microcephaly • Viral Infections


Books and Articles

Brasil, Patricia, et al. “Zika Virus Infection in Pregnant Women in Rio de Janeiro—Preliminary Report.” New England Journal of Medicine, E pub March 4, 2016. http://www.nejm.org/doi/full/10.1056/NEJMoa1602412 (accessed March 13, 2016).

Korman, Caroline, “How Zika Virus Can Spread.” New Yorker, February 1, 2016. http://www.newyorker.com/tech/elements/how-zika-virus-can-spread (accessed March 12, 2016).

McNeil, Donald, Catherine Saint Louis, and Nicholas St. Fleur. “Short Answers to Hard Questions about Zika Virus.” New York Times, June 24, 2016. http://www.nytimes.com/interactive/2016/health/what-is-zika-virus.html?_r=0 (accessed June 30, 2016).


U.S. Centers for Disease Control and Prevention. “Zika Virus.” http://www.cdc.gov/zika (accessed March 12, 2016).

World Health Organization. “Zika Virus.” http://www.who.int/mediacentre/factsheets/zika/en (accessed March 12, 2016).


Centers for Disease Control and Prevention. 1600 Clifton Rd., Atlanta, GA 30329. Toll-free: 800-232-4636. Website: http://www.cdc.gov (accessed May 12, 2016).

National Institute of Allergy and Infectious Diseases. 5601 Fishers Lane, MSC 9806 Bethesda, MD 20892-9806. Toll-free: 866-284-4107. Website: http://www.niaid.nih.gov (accessed May 12, 2016).

* Guillain-Barré (GHEE-yan ba-RAY) syndrome is a rare disorder characterized by progressive symmetrical paralysis and loss of reflexes, usually beginning in the legs. The paralysis characteristically involves more than one limb (most commonly the legs). Some people recover completely in several months, but it is fatal for 5 percent of those with the syndrome.

* antibodies are proteins produced by the immune system to fight infection or rid the body of foreign material. Specific antibodies are produced in response to different infectious agents and can only inactivate that particular infection.

  This information is not a tool for self-diagnosis or a substitute for professional care.