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The Centers for Disease Control and Prevention has reported a Zika-related death in Puerto Rico—the first fatality in the United States caused by the mosquito-borne virus. The patient, a man in his 70s, developed a severe case of thrombocytopenia that caused internal bleeding, according toThe Washington Post.

While Zika deaths are still rare, this case “highlights the possibility of severe cases, as well as the need for continued outreach to raise health care providers’ awareness of complications that might lead to severe disease or death,” according to the CDC.

In December, Puerto Rico became the first U.S. jurisdiction to report local transmission of Zika virus, meaning that mosquitoes in that area are infected. Up until that point, U.S. residents only became infected when traveling abroad. The Puerto Rico Health Department reported that it now has 707 confirmed cases of the virus.

The CDC is encouraging people, particularly in Puerto Rico, to continue preventing mosquito bites by using repellents, wearing long-sleeved shirts and pants, and ensuring that windows in their homes are shut tight.

As the virus can also be spread through sexual transmission, the CDC advises people to properly use condoms or practice abstinence. These measures can also reduce the risk of microcephaly—a birth defect in which the baby’s head is smaller than expected.

The Pan American Health Organization issued an alert about the first confirmed Zika virus infections in Brazil last May. Since then, outbreaks have occurred in more than two dozen countries. In February, the World Health Organization declared it a public health emergency of international concern.