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.
Men can infect their sexual partners and pregnant women can pass the virus to their unborn children. No case has yet been reported of transmission through breast milk. The virus has been found in sperm up to 93 days after symptoms began, but men may carry the virus much longer. Therefore, the CDC recommends men wear condoms during sex for six months if symptomatic and at least eight weeks after returning from a Zika infected area, if asymptomatic. One case has been reported where a woman apparently transmitted the virus through vaginal fluid to a male partner.
Blood donors who have traveled in areas where Zika is mosquito-borne are deferred. In Puerto Rico it is recommended that blood and blood products be imported from the US or screened by an FDA-approved nucleic acid test (RT-PCR from Roche Molecular Systems). Plasma and platelets can also be treated with pathogen-reduction technology. 68 out of 12,777 blood donations in Puerto Rico tested positive between April 3 to June 11, 2016 and this number has been increasing since then. It would also be possible to be infected through tissue or organ transplantation.
Since Zika virus is now endemic in southern Florida, the FDA recommends all blood and blood products be tested in the US.
The CDC has made the following recommendation, "Even if they do not feel sick, travelers returning to the United States from an area with Zika should take steps to prevent mosquito bites for 3 weeks so that they do not spread Zika to uninfected mosquitoes."
New York City (NYC) has reported 233 cases of Zika between 1/19/2016 and 6/24/2016. All cases were in persons who returned to New York after traveling to the Dominican Republic (140 cases), Puerto Rico (20 case), Guyana (14 cases), and other countries (65 cases). 24 of those are pregnant. Ae. albopictus is a potential competent Zika vector present in NYC.
Local mosquito-transmitted cases of Zika not related to travel or sexual transmission have been reported from Florida. Texas and other Gulf Coast states will probably also have local mosquito-borne disease soon.