A new report from the Centers for Disease Control and Prevention says the Zika virus does cause birth defects like microcephaly, but not in every case.
Researchers have been trying to determine the link between Zika and microcephaly since the mosquito-carried virus began to spread quickly early last year, especially in Central and South America. Microcephaly is a birth defect that causes a baby's head to be abnormally small.
CDC Director Dr. Thomas Frieden said in a press release, "There is no longer any doubt that Zika causes microcephaly."
The CDC noted "no single piece of evidence provides conclusive proof," but said multiple studies backed up the CDC research.
There were other theories about what was causing the rise in microcephaly cases. One report suggested pesticide-contaminated water could be to blame.
And some Brazilians believed that vaccines were causing Zika.
Currently, there's no cure for the Zika virus or microcephaly.
But researchers who worked on an Ebola vaccine say human trials for a Zika vaccine could begin as early as this summer.