As the Zika virus continues to spread across Brazil, health organizations fear the spread of infection from the mosquito-borne illness that can cause dangerous birth defects. This is why the U.S. Centers for Disease Control and Prevention (CDC) released an official travel advisory for pregnant American women to stay away from the Summer Olympic Games in Rio de Janeiro from August 5 to 21, The New York Times reports.

Since February 2015, the Zika virus has infected as many as 1.5 million people in Brazil, prompting scientists to study whether the illness is responsible for causing more than 4,000 cases of microcephaly. This rare condition causes infants to be born with abnormally small heads that can lead to lifelong developmental delays. In addition, researchers linked the Zika virus to miscarriage, brain damage and acute illness in adults, with symptoms including a fever, body aches and fatigue lasting up to a week.

The CDC’s travel advisory also recommends that pregnant women with a male partner who plan to attend the Olympics either use condoms or abstain from sex for the duration of the mom-to-be’s pregnancy. This is because recent reports found that the Zika virus can be transmitted sexually.

The CDC advised pregnant women who decide to visit Brazil anyway to adhere to the following precautions: use insect repellent to avoid mosquito bites, cover exposed skin and stay indoors as much as possible.

For more information about the Zika virus and how to avoid infection, click here.