Since local mosquito-borne transmission of Zika virus was first reported in Brazil in early 2015, the virus has spread rapidly, with active transmission reported in at least 61 countries and territories worldwide, including the United States. Zika virus infection during pregnancy is a cause of microcephaly and other severe brain anomalies. The virus is transmitted primarily through the bite of an infected Aedes mosquito, but other routes of transmission include sexual, mother-to-fetus during pregnancy, mother-to-infant at delivery, laboratory exposure, and, possibly, transfusion of blood products. Most persons with Zika virus infection are asymptomatic or have only mild symptoms; hospitalizations and deaths are rare. When symptoms are present, maculopapular rash, fever, arthralgia, and conjunctivitis are most common. Zika virus testing is recommended for persons with possible exposure (those who have traveled to or live in an area with active transmission, or persons who had sex without a condom with a person with possible exposure) if they have symptoms consistent with Zika virus disease. Testing is also recommended for pregnant women with possible exposure, regardless of whether symptoms are present. Treatment is supportive, and no vaccine is currently available. The primary methods of prevention include avoiding bites of infected Aedes mosquitoes and reducing the risk of sexual transmission. Pregnant women should not travel to areas with active Zika virus transmission, and men and women who are planning to conceive in the near future should consider avoiding nonessential travel to these areas. Condoms can reduce the risk of sexual transmission.