“The bottom line is we’re seeing a steady increase,” Dr. Frieden said.

He stressed that pregnant women are the chief priority in the fight against Zika. The virus has been shown to cause microcephaly, a birth defect marked by small head size and underdeveloped brains, as well as other malformations. It is yet not clear whether babies exposed to Zika in the womb with no obvious birth defects will have developmental issues later on.

In Brazil, authorities have confirmed more than 1,400 cases of microcephaly in babies whose mothers were exposed to Zika during pregnancy.

Dr. Frieden said the test was highly sensitive, and all blood donations that test positive are removed from the blood supply. Donors who test positive for Zika, an infection that often has no symptoms, are given information on how to avoid spreading the infection to others.

Although Zika is largely a mosquito-borne virus, it can be transmitted sexually and through blood donations.

Dr. Frieden said there have been no confirmed cases of Zika spread through blood transfusions anywhere in the United States or Puerto Rico or any other U.S. territory, although there has been at least one report of Zika spreading through a blood transfusion in Brazil.

The Obama administration has requested $1.9 billion in emergency Zika funding. The Senate approved $1.1 billion of that request, but the House of Representatives voted to allocate $622.1 million financed through cuts to existing programs, such as for Ebola, which U.S. health officials have called inadequate and short-sighted.