Zika fever is an illness caused by the Zika virus. The Zika virus belongs to the genus Flavivirus, like dengue fever and chikungunya. However, Zika fever is usually not as bad as those illnesses. Most people who get the Zika virus (60-80%) have no symptoms.

People who do have symptoms usually have a low fever, conjunctivitis, joint pain (mainly in the hands and feet), and a rash. The rash often starts on the face and then spreads to the rest of the body. Usually, the symptoms are not very bad, and get better after 2 to 7 days.

Starting in 2015, there has been an outbreak of Zika virus (meaning that many people have gotten the virus) in Brazil. Scientists think that when a woman with the Zika virus is pregnant, she can give the virus to her fetus. Scientists think this can cause microcephaly, a birth defect that causes a baby to have a smaller head than usual. This can cause intellectual disability and other problems in the brain, like seizures. Because of this, in January 2016, the United States Centers for Disease Control and Prevention warned pregnant women not to travel to Brazil, Colombia, El Salvador, French Guiana, Guatemala, Haiti, Honduras, Martinique, Mexico, Panama, Paraguay, Suriname, Venezuela, or Puerto Rico, because of the risk of getting the Zika virus there.

In places where the Zika virus lives, people are more likely to have birth defects, neurological problems like Guillain-Barré syndrome, and autoimmune diseases.