An Aedes aegypti mosquito inside a test tube at a research facility in Mexico. The mosquito is a carrier of Zika virus. (Daniel Becerril/Reuters)

THE REPUBLICAN-controlled Congress has wasted entirely too much time sitting on President Obama’s request for emergency funding to combat the arrival of the Zika virus to the mainland United States. The National Governors Association, not exactly an alarmist group, declared that “the nation is on the threshold of a public health emergency.” Anthony S. Fauci, director of the National Institute of Allergy and Infectious Diseases, says that Puerto Rico, a U.S. territory where the virus is already on the move, “is on the precipice of a really serious disaster.” Now that Congress has returned from its recess, it is time to buckle down and approve the president’s request for about $1.9 billion in emergency funding, or something close to it.

Secretary of Health and Human Services Sylvia Mathews Burwell told us this week that, although the administration has already shifted about half a billion dollars from fighting Ebola to the battle against Zika, that is not enough. There is no vaccine ready nor any known effective therapy to stop Zika. Without the full emergency funding, she said, research on creating a vaccine and work on badly needed diagnostics will slow, while surveillance and tracking of those sickened will be hampered. Although Zika causes only mild symptoms in most cases, thousands of babies, if their mothers are infected during pregnancy, could be vulnerable to serious birth defects. The virus causes fetal neural abnormalities such as microcephaly.

The time to prepare for the onslaught of virus-carrying Aedes mosquitoes was yesterday — and yet Congress has stalled the president’s February funding request. The House Appropriations Committee chairman, Harold Rogers (R-Ky.), has claimed the proposal lacked enough specifics. Here are some specifics: In Puerto Rico, where 631 cases of Zika have already been recorded, the virus is spreading rapidly from direct transmission. Although all 472 cases in the continental United States have come from travelers, Florida, the Gulf Coast and California also may be vulnerable to local transmission once the weather warms.

Where it hits, Zika is not easy to deal with. “Vector control” — fighting the mosquitoes, to dampen the virus’s spread — is the front line of defense, but that job is handled by a crazy quilt of state and local government offices. Warning the public, creating and scaling up diagnostics, and carrying out spraying and other prophylactic measures are all vital, and these take time to prepare, time that is being lost on Capitol Hill.

Vaccine development also requires a long lead time. Dr. Fauci told us the first vaccine candidate will begin an initial clinical trial of about 80 people in September, and if all goes well, could be ready for approval in 2018. This will be especially important if the Zika virus remains a threat at that time. We can’t know now if that will be the case, but it makes good sense to fund vaccine development robustly at this early stage.

We are told that House Appropriations Committee staff, still dissatisfied with the administration’s request, are working up their own package. That’s encouraging — but they must not dally. It is long past time for Congress to deliver the money needed to fight a virus that, if unchecked, could ruin thousands of young lives.