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Zika Definitely, Without a Doubt Causes Microcephaly

The CDC isn't mincing words anymore.

When the Zika virus first started spreading through the Americas, in 2015, health officials noticed a disturbing trend—more and more babies in affected areas were being born with microcephaly, a birth defect that causes abnormally small heads and underdeveloped brains. But that side effect had never been linked to this particular virus before, and at that point, nobody could say for certain that Zika was definitely to blame.

On April 13, after months of research, the US Centers for Disease Control finally confirmed what people have long suspected: Zika, which is carried by Aedes mosquitoes, causes microcephaly and other severe brain defects in developing fetuses, these scientists say. And it could cause other side effects, too, that we don't even know about yet.

In the new paper, published in the New England Journal of Medicine, CDC experts combed through many studies on the Zika virus to reach their conclusions. Their report notes that there wasn't one smoking gun—rather, that all the evidence taken together makes a strong case for the fact that Zika causes microcephaly and other problems.

Then again, some pregnant women who pick up Zika seem to go on to have healthy babies, although the reasons why aren't understood.

"This study marks a turning point in the Zika outbreak. It is now clear that the virus causes microcephaly," CDC director Dr. Tom Frieden said in a statement. "We are also launching further studies to determine whether children who have microcephaly born to mothers infected by the Zika virus is the tip of the iceberg of what we could see in damaging effects on the brain and other developmental problems."

In other words, there could be more unpleasant surprises to come.

Although doctors, health officials, and families in affected areas have long believed that the Zika virus can cause microcephaly, nobody could say for sure because—until very recently—the virus was thought to be relatively harmless, so nobody had studied it in much detail. Now that it's become clear how dangerous it is, scientists are doing everything they can to understand the virus, and to figure out how to treat it: they're making mouse models to study it in the lab, determining its structure, and working on a vaccine.

For now, the CDC isn't changing any of its guidelines around Zika. Instead, it hopes that today's declaration will drive money and efforts towards research and prevention.

That's increasingly important because the weather's getting warmer, and the mosquitoes are coming out. Local health authorities are bracing themselves, fearing that the Zika virus could begin to spread locally within the US.