Health officials are warning would-be travelers about the spread of a mosquito-borne illness that might be causing a growing number of children to be born with smaller-than-usual heads.

Late last week, the Centers for Disease Control and Prevention (CDC) issued a travel notice for Puerto Rico after the island reported its first locally contracted case of the virus. The advisory encourages travelers to take proactive steps to prevent mosquito bites, like wearing insect repellant and long clothing. The CDC has issued similar warnings for Mexico, Central America and South America, as several other countries have said the virus has sickened humans there.

Zika often causes people to experience fever, joint pain, rashes and red eyes for up to a week, the CDC says, but the illness seldom results in hospitalization. However, the Brazilian Ministry of Health recently said it was investigating about 3,000 cases of microcephaly, a neurological disorder in which children are born with unusually small heads and underdeveloped brains.

Before this year, the ministry said it usually saw about 150-170 annual cases of microcephaly. But since the Zika virus arrived in May, the ministry said the number of suspected cases have surged to 2,975. Authorities are currently examining if there’s a conclusive link between the two.

The CDC said test results have shown that some Brazilian babies with microcephaly were infected with Zika, but other babies developed microcephaly and tested negative for the virus. There are other possible causes for the defect, so it will take some time to determine the culprit. Still, the CDC says woman who are pregnant should be extra careful to prevent mosquito bites.

There isn’t any vaccine that can prevent the virus or any medicine that can treat it, according to the CDC. Because the mosquitos that transmit Zika live all across the planet, the organization says it’s likely that the virus could spread to new regions where illnesses have yet to be reported.

The virus can remain in the bloodstreams of infected individuals for about a week, according to the CDC, during which time it’s extra important to avoid mosquito bites to prevent transmission. While the organization reports no one has acquired Zika in the United States, there has been at least one instance where a traveler got sick while visiting a foreign country and returned home.