To fight the mosquito-borne Zika virus, the US health officials have appealed again for emergency funding, after meeting resistance from Republican lawmakers who want to use money previously allocated for Ebola.
In February, President Barack Obama's administration asked Congress for $1.9 billion to boost preparedness and response to Zika, a poorly understood virus which has been linked to a surge in birth defects in Brazil and is expected to infect hundreds of thousands of people in the US territory of Puerto Rico by year's end.
But the request has stalled in the House Appropriations Committee, where Republicans say the administration should put $2.7 billion in unspent money for fighting the Ebola virus towards Zika preparedness.
Frieden hosted a summit on the Zika virus Friday, drawing 178 representatives from health departments, dozens of non-governmental organizations and government agencies to Atlanta, along with some 2,000 people watching a webcast about the latest scientific data and prevention strategies.
He and other government officials also held a conference call with reporters to discuss the challenges of facing down the first-known mosquito-borne virus that appears to infect the fetal brain, though officials have yet to confirm whether Zika causes microcephaly, a life-long malformation in which an infant's brain is much smaller than normal.
Zika has spread quickly to more than 30 places in Latin America and the Caribbean since last year.
In the mainland United States, dozens of pregnant women have been infected after traveling to parts of the world where Zika is present, and two pregnant women were infected with Zika through sex, he said.
Asked to quantify the risk to the United States, Frieden declined to predict how many Zika cases might be seen in the continental United States in the coming months.
"We don't want to speculate about what may happen. We want to maximize out preparedness for what we can prevent from happening."
Puerto Rico on the Brink
Even though the Zika virus was first identified in 1947, the outbreak of birth defects has taken health authorities by surprise.
Frieden said it remains unclear how to advise women about the actual risk of fetal abnormalities.
"There are just too many unknowns," he said.
Frieden has warned that Puerto Rico is on the brink of a massive Zika crisis, and may have thousands of pregnant women infected in the coming months.
The $1.9 billion requested includes $828 million for the CDC and would be used to accelerate vaccine research, develop a long-term strategy to prevent Zika, expand lab capacity and testing, boost mosquito control, and support low-income pregnant women, officials said.
There is no vaccine on the market to prevent Zika and often the virus does not cause symptoms in adults who are infected. When it does, the illness may be mild and includes fever, rash and red eyes.
In February, the House Appropriations Committee responded to Obama's request by pointing to $1.4 billion of unused Ebola funds within the Department of Health and Human Services, and another $1.3 billion unused in the Department of State and USAID coffers.
"Some in Congress have just suggested we use the money we had to fight Ebola to fight Zika," said Amy Pope, White House Deputy Homeland Security Advisor.
"Even if we take money from other pots, even if we make these very hard choices, the money we have now is not enough," she added.
"We cannot wait until we see widespread transmission in the United States before taking steps to prepare. It is just too late at that point."