A disease until now found only in rats has made the leap to humans, worrying health experts.
The hepatitis E virus recently infected a Hong Kong man - the first person ever confirmed to have the disease, according to researchers at the University of Hong Kong.
"This study conclusively proves for the first time in the world that rat HEV can infect humans to cause clinical infection," they said in a statement.
Rat hepatitis E is "very distantly" related to the human hepatitis E, a liver diseae that affects 20 million people worldwide. Symptoms include fever, vomiting and jaundice.
The 56-year-old Hong Kong man who caught the rat version probably got it through food infected by rat droppings, the researchers told the South China Morning Post, saying the development had "major public health significance".
Luckily, he was able to be cured of the disease with medication used to fight human hepatitis E.
"We don't know if in future there will be a serious outbreak of the rat hepatitis E virus in Hong Kong," Prof Yuen Kwok-yung, a microbiologist at Hong Kong University, said. "We need to closely monitor this issue."