“The Zika virus causes birth defects,” says the Center for Disease Control Wednesday. If women have the Zika virus and are pregnant, it is likely the virus is the cause of an under-developed brain.

The CDC said there could be an environmental exposure that causes birth defects. “We started using criteria about a month ago to see which ones had been met and which ones had not been met. We wanted to do this in a systematic and calculated way,” said CDC’s Dr. Sonja Rasmussen.

The virus is being spread by mosquitoes in Florida, but The CDC says that they are not ready to confirm that the mosquito transmits Guillain-Barré syndrome.

Not all women who had the Zika virus while pregnant will have birth defects.

Earlier outbreaks in the Pacific Islands showed there was no sign of birth defects related to the virus, which there is no vaccine for.

According to the CDC, “Zika is spread through mosquitoes but can also be spread through sexual intercourse. The best way to prevent Zika is to prevent all mosquito bites. Mosquito bites can be prevented by using bug spray. Be sure to wear long sleeves and long pants and stay in air-conditioned places. Pregnant women should not travel to places that are infected with the virus.”