There's no longer any doubt that the Zika virus causes birth defects, federal health officials said Wednesday.
The evidence has been piling up and there really had not been any doubt that Zika was causing horrific brain damage to unborn babies. But the Centers for Disease Control and Prevention made it official Wednesday in a publication rushed into the New England Journal of Medicine.
"It is now clear … that Zika does cause microcephaly," CDC Director Dr. Tom Frieden told reporters in a briefing. "We believe the microcephaly is likely to be part of a range of birth defects."
Zika's been spreading in Brazil and since last year officials there noted a startling increase in the number of cases of microcephaly, a birth defect caused by an underdeveloped brain and marked by a notable small head.
There were doubts at first that Zika, once thought to be a pretty harmless virus, could actually be causing birth defects. Until Brazil started raising the alarm, doctors didn't think it even made most people sick. Mosquito-borne viruses had never before been known to cause birth defects, although other viruses, such as rubella, are notorious for causing them.
But study after study has shown the virus gets into the developing fetus's brain, killing brain cells, stopping it from growing and, often, killing the fetus. And it seems to do so at all stages of pregnancy, not just in the first trimester, as most other viruses do.
"There is no longer any doubt that Zika causes microcephaly," Frieden said.
The CDC, the World Health Organization, the National Institutes of Health and other agencies have been sounding the alarm about Zika for months. Not only is it spreading across Latin America and the Caribbean; it is being carried into the United States by travelers.
It can be spread sexually and in the summer, when mosquito season starts, it is likely to cause local oubtreaks. It's already caused outbreaks in the U.S. territories of Puerto Rico, the U.S. Virgin Islands and Samoa.
"Reducing exposure to mosquitoes for everyone where the virus is circulating is important," Frieden said.
The Obama administration has asked Congress to appropriate $1.9 billion to fight Zika, but House Republicans say they don't want to spend any new money until they are sure all other available funds are used.
In an unusual retaliation, the White House has freed up officials such as Frieden and the NIH's Dr. Tony Fauci to plead for the cash and to outline what other programs will be cut if they don't get it.
White House press secretary Josh Earnest released a new salvo Wednesday, saying a bill aimed at speeding up vaccine development wasn't anywhere near enough to help.
"In some ways, it's akin to passing out umbrellas in advance of a hurricane," Earnest told the White House briefing.
"It's not going to do anything to help local communities across the country," he added. "It doesn't include any funding."
Much more study is needed, as well as a vaccine to prevent Zika infection, drugs to treat it and better mosquito control measures, officials say.
"Never before have we seen an illness spread by mosquitoes leading to a birth defect," Frieden said.
And the CDC's Dr. Sonja Rasmussen says it's important to find out fast what else Zika might be doing.
"This doesn't mean that we have all the answers," she told reporters. Doctors still don't know how many pregnant women who get Zika will have babies with birth defects. They don't know what all birth defects there will be, and whether some might show up in childhood or even adulthood. They don't even know if a woman has to have symptoms of Zika infection for it to affect her baby.
And there's more evidence that Zika can be passed person to person, and not only through mosquito bites. The CDC's confirmed sexual transmission of Zika and now tells pregnant women to use condoms when having sex if their partner has been to a Zika-affected area.
In a letter to the New England Journal of Medicine, French doctors reported on a case of sexual transmission of the virus last February, after a man traveled to Rio de Janeiro, got infected, and then had sex with a woman in France after he got better.
"We cannot rule out the possibility that transmission occurred not through semen but through other biologic fluids, such as pre-ejaculate secretions or saliva exchanged through deep kissing," Dr. Eric D'Ortenzio of France's INSERM national research institute and colleagues wrote.
Zika's been found in saliva, and the team said people need to be warned that there's a possibility they could catch it that way, they said.
This article first appeared on NBCNews.com.