WHEN MEMBERS of Congress visit their districts over the Memorial Day recess, we hope they keep in mind a warning from Tom Frieden, director of the Centers for Disease Control and Prevention. The Zika virus, for which there is no effective vaccine or therapy, can infect a person in a flash, he said during a visit to The Post on Thursday. “You can get a mosquito bite and your life changes,” he said. The virus is mild for most people but in pregnant women can lead to fetal neural defects, and the prospect that local transmission will soon begin in the United States is very real.

The Republican congressional leaders, Senate Majority Leader Mitch McConnell (Ky.) and House Speaker Paul D. Ryan (Wis.), have vowed to end gridlock on Capitol Hill. But in response to Zika, they have inexcusably dithered. President Obama asked for nearly $1.9 billion in emergency funding in February. When Congress failed to act quickly, the administration redirected existing funds in order to scale up an initial response. After much delay, the House and Senate passed widely divergent bills, neither entirely fulfilling the president’s request. The Senate made a serious start with what Sen. Patty Murray (D-Wash.) calls a $1.1 billion “down payment,” with no offsets, which was supported by all Democrats and slightly fewer than half the Republicans. House Republicans wasted weeks and weeks quibbling over whether Mr. Obama’s request was detailed enough, then approved a $622 million bill that would raid accounts for Ebola and other important work, such as emergency preparedness, and would last only through September. The House bill is woefully inadequate. Even more irresponsible was the GOP diversionary tactic to push through a bill to weaken the Clean Water Act under the guise of a Zika response. Congressional Republicans are playing games with a public-health emergency.

The enemy here is a formidable one. As Mr. Frieden told the National Press Club on May 26, the Aedes egypti mosquito that carries Zika is “the cockroach of mosquitoes. It lives indoors and outdoors. It bites through the daytime and in the nighttime. Its eggs can last more than a year. They can hatch in a drop of water. . . . When they take a blood meal, they will often bite four or five people at once. So they’re capable of rapidly spreading the infection. There is no example of effective control of this mosquito in the modern era.”

To fight an outbreak requires resources and time. Congress is undermining the effort on both counts. Vaccine development, now underway, is a long-term project; surveillance is a multi-year endeavor; creating rapid diagnostics is a major undertaking; mosquito control is fragmented in local governments, often uneven and urgent. Those on the front lines need to know — now — that programs to fight Zika will not start and stop. When members of the House and Senate return next week, they must immediately go to conference and pass emergency supplemental funding, along the lines of the Senate bill, to enable a determined and serious battle against the Zika virus. Scientists and public-health experts know what to do. Why are Republicans in Congress impeding their necessary work?