Authorities in China have stepped up precautions after a city in the Inner Mongolia autonomous region confirmed one case of bubonic plague.
According to state reports, the Bayannur patient - a herdsman - is in quarantine and in a stable condition.
Officials said they were also investigating a second suspected case, according to China's Global Times.
The bubonic plague was once the world's most feared disease, but can now be easily treated.
The first case was reported as suspected bubonic plague on Saturday at a hospital in Urad Middle Banner, in Bayannur city. It is not yet clear how or why the patient might have become infected.
The second suspected case involves a 15-year-old, who had apparently been in contact with a marmot hunted by a dog, a tweet from Global Times said.
A level 3 alert, which forbids the hunting and eating of animals that could carry plague and calls on the public to report suspected cases, has been put in place until the end of the year.
What is bubonic plague?
Bubonic plague, caused by bacterial infection, was responsible for one of the deadliest epidemics in human history - the Black Death - which killed about 50 million people across Africa, Asia and Europe in the 14th Century.
There have been a handful of large outbreaks since. It killed about a fifth of London's population during the Great Plague of 1665, while more than 12 million died in outbreaks during the 19th Century in China and India.
But nowadays it can be treated by antibiotics. Left untreated, the disease - which is typically transmitted from animals to humans by fleas - has a 30-60% fatality rate.
Symptoms of the plague include high fever, chills, nausea, weakness and swollen lymph nodes in the neck, armpit or groin.
Could there be another epidemic?
Bubonic cases are rare, but there are still a few flare-ups of the disease from time to time.
Madagascar saw more than 300 cases during an outbreak in 2017. However, a study in medical journal The Lancet found less than 30 people died.
In May last year, two people in the country of Mongolia died from the plague, which they contracted after eating the raw meat of a marmot - the same type of rodent the second suspected case came into contact with.
However, it's unlikely any cases will lead to an epidemic.
"Unlike in the 14th Century, we now have an understanding of how this disease is transmitted," Dr Shanti Kappagoda, an infectious diseases doctor at Stanford Health Care, told news site Heathline.
"We know how to prevent it. We are also able to treat patients who are infected with effective antibiotics."