Jackeline, 26, holds her son Daniel who is 4-months old and born with microcephaly, in front of their house in Olinda
Thomson Reuters
The Centers for Disease Control and Prevention (CDC) has officially said that the Zika virus is a cause of microcephaly and other birth defects.

The confirmation was published Wednesday in a report in the New England Journal of Medicine. The conclusion was drawn from existing evidence, including recently published studies. Microcephaly is a condition in which the head is born abnormally small, and it can be caused by other factors beyond Zika.

"This study marks a turning point in the Zika outbreak. It is now clear that the virus causes microcephaly," CDC director Tom Frieden said in a news release. "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."

This conclusion is big for understanding the gravity of Zika. Before now, researchers have only been able to confirm a link between microcephaly and Zika, but not definitively say there's a causal relationship. Now that Zika is officially understood to be a cause of microcephaly, researchers can now explore what other damage is being done. Often, babies born with abnormally small heads suffer from underdeveloped brains and other serious complications including vision problems, though there's some variability there.

The Zika virus is a mosquito-borne infection that has been spreading around the Americas and Pacific islands over the few months. There have also been instances in which it spreads sexually. Once infected with Zika, only about 20% of people ever show symptoms, which most commonly include fever, rash, joint pain, and red eyes. And although Zika is a cause of microcephaly and birth defects, the CDC stressed that not all women who get infected with Zika while pregnant will have babies with these problems.