Zika virus, a flavivirus that is recently emerging in the Americas, may cause mild symptoms such as: fever, skin rash, conjunctivitis, muscle and joint pain or headache. More severe neurological complications have been documented, such as Guillain-Barré syndrome and microcephaly or other congenital brain abnormalities in the developing fetus of an infected pregnant female. Zika virus was first identified in rhesus monkeys in the Zika forest of Uganda in 1947. It has since expanded across the South Pacific islands into South America, Central America and the Caribbean. The virus is most commonly transmitted to humans through mosquito bites; but may also be transmitted by sexual contact, other contact, laboratory acquisition or blood transfusion. Zika virus nucleic acids (RNA) are detected using an investigational NAT assay. Investigational NAT in mini-pools of 16 was introduced in June 2016 in 5 southeastern states and by October to 10 states, all performing testing on individual donations. Testing expanded to all Red Cross regions by November 2016. Since December 2016, all Red Cross donations are tested individually by NAT. NAT-reactive donations are further tested by various NAT methods including PCR; antibody testing is also performed to confirm infection.