In 2021, five people in the United States died of rabies — the highest number in a decade.

Rabies is a viral disease that infects the central nervous system of mammals. It is most often transmitted via the bite of a rabid animal — typically bats, skunks, raccoons, and foxes — and is usually fatal within a few weeks of symptoms starting. Death can be prevented by receiving a series of five shots that are administered within two weeks of exposure.

In a report released Thursday, the Centers for Disease Control and Prevention said that three of the 2021 deaths could have been prevented, but the victims — who were all bit or scratched by bats — did not want the shots. One of the men who died, an 80-year-old from Illinois, refused the shots due to a fear of vaccines. The other two, a man from Idaho and a boy from Texas, didn’t think the bats broke their skin, and felt the shots weren’t necessary.

In those cases, they “either trivialized the exposure [to bats] or they didn’t recognize the severity of rabies,” CDC rabies expert Ryan Wallace told The Associated Press. The other two cases involved a man from New York who was bit by a rabid dog in the Philippines and died after returning home, as well as a Minnesota man who received the shots, but due to an undiagnosed issue with his immune system, they were not as effective.

Symptoms of a rabies infection include anxiety, confusion, insomnia, paralysis, hallucination, fear of water, and difficulty swallowing. In the United States, about 60,000 Americans are treated annually after possibly being exposed to rabies. The last time five Americans died of rabies was in 2011, CDC officials said, and there were no deaths reported in the U.S. in 2019 or 2020.