The virus is spread to people primarily through the bite of infected Aedes aegypti and sometimes Ae. albopictus. These mosquitoes live in a wide range of climates and aggressively bite people during the daytime and sometimes at night. Anyone who lives or travels where Zika is endemic or epidemic can be infected. Many illnesses are asymptomatic; therefore, travelers may return to the United States infected, where they can then spread Zika.
The Zika virus can be passed through sexual activity. It has been detected in vaginal fluid, semen, saliva, urine, and breast milk. The virus has been found in sperm up to 93 days after symptoms began. Therefore, the CDC recommends men wear condoms during sex for three months, and women wear condoms for two months after returning from a Zika-infected area.
Pregnant patients can pass the virus to their unborn children. While the Zika virus has been found in breast milk, no transmission has yet been confirmed through breast milk.
Blood donors who have traveled in areas where Zika is mosquito-borne are deferred. There have been no confirmed cases of Zika virus being spread by blood transfusion in the US. However, transmission through platelet transfusions has been documented in Brazil. Due to the potential risk, the FDA implemented Zika virus screening of blood donations in 2016. In 2021, due to the global decline in the incidence of the virus, the FDA then removed the requirement.
Local mosquito-transmitted cases of Zika not related to travel or sexual transmission have been reported in the United States previously. There have been no reported cases since 2018.
