Dr. Tom Frieden has dealt with a number of epidemics during his seven-year tenure as director of the Centers for Disease Control. But the rapidly spreading Zika virus, the terrifying birth defects it causes and Congress' inexplicable foot-dragging on funding anti-Zika efforts has him feeling downright desperate.

"Imagine that you're standing by and you see someone drowning, and you have the ability to stop them from drowning, but you can't," Frieden told a packed room of reporters and potential donors at the National Press Club on Thursday. "Now multiply that by 1,000 or 100,000. That's what it feels like to know how to change the course of an epidemic and not be able to do it."

Frieden displayed an unusual level of vulnerability for a public official on Thursday as he begged for more private sector donations and implored a divided Congress to move faster to combat the "extraordinary and unusually urgent" crisis.

"I'm often asked how I feel as CDC director," he said. "In the heat of the moment, you're mostly concerned about getting the job done.... but for me, when faced with emergencies like this, the greatest emotion has been frustration."

Zika, a mosquito-borne virus that can cause babies to be born with unusually small and deformed heads, is rapidly spreading through South and Central America and has already infected at least 1,500 people in the United States. Frieden said the need to stop Zika in its tracks in the U.S. is more urgent than ever right now, as the weather grows hotter and more mosquito-friendly. But the challenges of combatting Zika are enormous and unusual.

First, diagnosing Zika is difficult. Four out of five people affected display no symptoms and the virus only stays in a person's blood or urine for one to two weeks. Second, the type of mosquito that transmits Zika is very difficult to control. Frieden said his researchers put a Zika-infected mosquito in a bottle coated with a strong insecticide, and the insect flew "happily" around the jar for hours.

"This is the cockroach of mosquitos," he said. "It lives indoors and out, bites in the day and in the night. The eggs can last more than a year, and they can hatch in a drop of water.... When they take a blood meal, they'll bite four or five people at once, so they're capable of rapidly spreading infection."

Frieden said the CDC needs funding to stop the epidemic in its tracks and protect pregnant women from becoming affected. The agency is trying to confront Zika in Puerto Rico, which because of its climate is particularly friendly to the mosquito, and to support women who choose not to get pregnant right now with effective modern contraceptives. The CDC also needs to develop retroactive tests so that pregnant women can discover whether they've been affected at any point in their gestation. But nearly four months after the CDC first asked for $1.9 billion in funding to deal with the epidemic, Congress just failed to pass a bill to give health officials the money they need and then left for a 10-day recess.

Frieden said his "jaw dropped" when he realized how long it would take Congress to move on the issue. "Three months in an epidemic is an eternity," he said.

The cost for treating just one baby with microcephaly, the birth defect caused by Zika, is estimated to be about $10 million. More than 300 pregnant women in the U.S. and its territories are confirmed to have Zika. Democrats skewered their Republican colleagues Thursday for trying to be "fiscally responsible" during a public health emergency.

"We can nickel and dime this if we want, but we do so at our own peril," said Rep. Jim McGovern (D-Mass.).

Frieden said Congress doesn't have time to bicker over funding right now.

"Memorial Day weekend heralds the start of mosquito season," he said. "We have a narrow window of opportunity to scale up Zika prevention measures, and that window is closing."