A national park in California has been evacuated and partially closed after a squirrel was found to be carrying the plague.
The disease, known as the "black death" or bubonic plague, is transmitted by fleas and killed around 25 million people in Europe in the 14th century.
Health officials in the US say the state's recorded four cases since 1984, and all of them have been non-fatal.
The plague is treatable with antibiotics, but if left untreated, it's fatal.
Dr Jonathan Fielding, LA County Public Health, spoke to NBC news in the US, confirming the case.
"We found one today, or yesterday, that in fact had plague … it had something like 12 fleas on it," he said.