The Food and Drug Administration is now recommending that all blood donations in the U.S. undergo testing for the Zika virus amid mounting concerns about how the disease is spread.

The agency’s sweeping recommendation Friday underscores concerns within the medical community about the still relatively unknown virus, which has spread rapidly by travel throughout the U.S. this summer. Zika-carrying mosquitoes have also arrived in parts of southern Florida this month.

“There’s tremendous uncertainty,” Dr. Peter Marks, head of the FDA’s Center for Biologics Evaluation and Research, said in a call with reporters.

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Several local blood banks, including in Tampa, Atlanta and Houston, have already implemented Zika screening procedures.

Those screenings have already prevented one Zika-positive person from donating blood, the FDA confirmed for the first time Friday.

Marks said that donor, whose blood tested positive before it could enter the national supply, was likely infected through travel.

Earlier this month, a team of researchers that included members of the American Red Cross said they believed a person infected with the Zika virus had spread the mosquito-borne disease to two people who received blood transfusions.

Blood banks in Florida and Puerto Rico will be required to “immediately” follow the new guidelines. Eleven other high-risk states, including Texas, New York and California, will have four weeks to comply. The rest of the country has 12 weeks.

Several members of Congress, led by Rep. Lloyd Doggett (D-Texas), first urged the FDA on July 20 — before the Zika was spreading locally in Florida — to implement mandatory screening for all blood donations.

“If we wait for the first confirmed locally transmitted Zika case to begin testing, we risk serious harm to the stability of our blood supply,” Doggett wrote to FDA Commissioner Dr. Robert Califf.

Implementing the additional screening will be costly, Marks acknowledged, though he declined to say how much the FDA will need to spend.

One local blood bank, the South Texas Blood and Tissue Center, has estimated that it costs just under $10 to screen each blood donor for Zika.

That cost would likely drop as Zika tests become more widely available, but it could still present a massive financial challenge for blood centers nationally that take in 6.8 million people every year.

Any new Zika-related expenses for the federal government are sure to become part of Congress's billion-dollar funding fight over the disease, which will resume when both chambers return after Labor Day.

“FDA understands the challenges that such recommendations may present,” Marks said. “It will require resources. We anticipate there to be some logistical issues.”

Part of the challenge is that there are only two blood tests available for Zika. Neither have been approved by the FDA yet, though blood centers have been given permission for “emergency use” of the tests.

The tests have been developed by Hologic and Roche Molecular Systems, the FDA said Friday.

More than 10,000 people in the U.S. and its territories have tested positive for the Zika virus, which is known to cause birth defects. Globally, there are more than 50 countries where the Zika virus is spreading by mosquitoes.

Public health officials have struggled to control the spread of the virus in part because it is spread beyond just mosquitoes.

Men can spread the disease through sex for several months after they’ve been infected, and women can spread it for several weeks, according to the Centers for Disease Control and Prevention.

—Updated at 12:16 p.m.