It’s not surprising that the congressional debate over funding the U.S. response to Zika has been hard to follow, given how much remains confusing about the virus itself. Which mosquitoes primarily transmit the infection in Latin America? Is the strain circulating there more dangerous than the one discovered in Africa nearly 70 years ago? How widely will it spread around the world? Most alarmingly, why is Zika-caused brain damage in fetuses and Guillain-Barré syndrome in adults only surfacing now?

Those unanswered questions, coupled with the mounting cases of serious harm caused by the virus in Latin America, explain why the Obama administration insists that it needs more to battle the epidemic than either the Senate or the House of Representatives this week agreed to spend. “Bottom line is, Congress needs to get me a bill,” President Barack Obama said today after a Zika briefing from the Department of Health and Human Services (HHS) Secretary Sylvia Burwell, Centers for Disease Control and Prevention (CDC) chief Tom Frieden, and Anthony Fauci, the point person from the U.S. National Institutes of Health (NIH). “It needs to get me a bill that has sufficient funds to do the job. … We've got outstanding scientists and researchers who are in the process of getting this done, but they’ve got to have the support from the public in order for us to accomplish our goal.”

On 22 February the White House requested $1.885 billion. The emergency appropriations proposal—which went into fine detail—gave $1.509 billion to HHS (which oversees NIH and CDC), $335 million to the U.S. Agency for International Development, and $41 million to the State Department.

It took legislators almost 3 months to mull over the request and respond. And what prompted Obama’s ire today is their stance on the amount of money to be allocated, when it can be spent, and the strings attached to its use.

Making the administration’s case in the Senate were Florida’s two senators, Republican Marco Rubio and Democrat Bill Nelson. In fact, their proposal for an emergency appropriation of $1.94 billion topped Obama’s initial request in part because it included a $144 million increase for research to develop a vaccine. Fauci, who heads the NIH’s National Institute of Allergy and Infectious Diseases, says the initial $130 million only covered 2016, whereas the Senate version also took 2017 spending into account. “It’s the difference between obligating and spending money,” Fauci says. The extra money also would bankroll some R&D for vaccine work on chikungunya and dengue, which are spread by the same mosquitoes as Zika, and epidemiological studies needed to lay the groundwork for field trials. But that plan, written as an amendment to a 2017 spending bill, never came up for a vote.

In the meantime, another bipartisan duo of senators had struck an agreement to spend $1.11 billion in emergency funds that would carry through 2017. The proposal, by senators Roy Blunt (R–MO) and Patty Murray (D–WA), cut out about $350 million specifically targeted to help Puerto Rico, a U.S. territory that currently is hard hit by Zika. It also reduced the NIH vaccine support to $200 million by nixing everything but the Zika vaccine money. Yesterday the Senate passed this bill as an amendment to a 2017 spending bill that funds transportation, housing, veterans affairs, and several other government agencies.

Over in the House, the head of the appropriations committee, Representative Harold Rogers (R–KY), offered $622 million this week for Zika support. But Rogers’s bill, which was not attached to any other spending measure, said all the money had to be spent by 30 September of this year. A chunk of the money, $352 million, would come from existing funding targeted to the country’s Ebola response, and the balance taps unspent HHS money set aside in part to combat emerging infectious diseases. Introduced on 16 May, it was approved 2 days later by the full House.

The House bill would require another vote by Congress for additional Zika funds in the 2017 fiscal year that begins on 1 October. CDC funding would plummet to $170 million from $828 million in the initial White House request, whereas NIH would get $30 million more for Zika vaccine R&D than the Senate bill provided. “I prefer $277 million,” says Fauci, referring to the administration’s updated request, “but if I can’t get that, I’d prefer $230 million and the ability to spend it beyond 2016.”

The next move is a conference between the House and the Senate to reconcile their differences. That process can take weeks, but Obama urged legislators to act much more quickly. Obama also said he’ll veto a final bill that provides only $622 million. “The House so far has approved about a third of the money that's been requested—except that money is taken from the fund that we’re currently using to continue to monitor and fight against Ebola,” Obama said today. “So, effectively, there’s no new money there; all that the House has done is said, you can rob Peter to pay Paul.”

While Congress debates how much to spend on Zika, CDC today announced that its new definition of a Zika “case” for pregnant women living in the United States and its territories has more than doubled the number. Instead of 112 pregnant women with confirmed cases of Zika—more than half in Puerto Rico—there now are 279, according to CDC’s Morbidity and Mortality Weekly Report published today. Although some have suggested the change may influence the political debate about Zika funding, it actually reflects CDC’s embrace of the interim case definition of Zika set by the World Health Organization: laboratory evidence of infection.

Previously, CDC adopted an “interim definition” set by the U.S.-based Council for State and Territorial Epidemiologists that said a case had to have at least one symptom, which included pregnancy complications. CDC changed its position after reports that asymptomatic pregnant women who had no obvious complications gave birth to babies with microcephaly and other brain defects.