(CNN) -- In 2018, infectious disease experts at the University of Hong Kong came across an unusual patient.
The 56-year-old man, who had undergone a liver transplant, was showing abnormal liver functions with no obvious cause.
Tests found that his immune system was responding to hepatitis E -- but they couldn't actually find the human strain of the hepatitis E virus (HEV) in his blood.
Hepatitis E is a liver disease that can also cause fever, jaundice and an enlarged liver. The virus comes in four species, which circulate in different animals; at the time, only one of these four was known to infect humans.
With tests for that human strain of HEV negative, the researchers redesigned the diagnostic test, ran it again -- and found, for the first time in history, rat hepatitis E in a human.
"Suddenly, we have a virus that can jump from street rats to humans," said Dr. Siddharth Sridhar, a microbiologist and one of the HKU researchers who made the discovery. It was such an unusual and unprecedented infection that the team wondered if it was a "one-off incident, one patient who was in the wrong place at the wrong time."
But then it happened again. And again.
Since that first study, 10 more Hong Kong residents have tested positive with rat hepatitis E, also known as rat HEV. The most recent case came a week ago; a 61-year-old man with abnormal liver function tested positive on April 30. And there might be hundreds more infected undiagnosed people out there, said Sridhar.