A man in Hong Kong is the first human to become infected with a type of hepatitis E infection that's only been seen in rats.

The 56-year-old man received a liver transplant at the University of Hong Kong's Queen Mary Hospital in May 2017, according to the South China Morning Post. Several months after the transplant, he began to have liver problems, and in September of that year, a test revealed that he had the rat version of hepatitis E. (Hepatitis is an inflammation of the liver.)

Doctors don't think the man got the virus from another human; the liver donor and the people who donated blood to the man all tested negative for the virus.

Still, it's unclear how the virus went from a rat to a human. One possibility is that the man ate food contaminated with rat droppings. The man lives next to a trash chute, where conditions aren't hygienic, the Post said. [10 Deadly Diseases That Hopped Across Species]

However, when investigators tested the mice in the area, as well as samples of drain water, they found no signs of the virus. It wasn't until they tested frozen samples of a rat from 2012 that had once roamed the area that they found a potential source: the dead rat had the virus, according to the Post. That finding, however, is a far cry from a clear answer.

How did this happen?

"Infectious diseases … can spread from rats to humans easily," said Dr. Amesh Adalja, a senior scholar at the Johns Hopkins Center for Health Security, who was not involved with the patient's case. This is thanks to "lots of commonalities between rats and humans," he said.

But one man's infection isn't necessarily a sign of an onslaught of rat-mediated hepatitis infections to come. "It's important to remember that this patient was a liver transplant patient, so he was probably more susceptible [to the virus] than an ordinary" person would be, Adalja said.

Organ transplant recipients must take drugs to suppress their immune systems so that the body doesn't reject the new organ. This makes them more susceptible to infection. Indeed, the combination of the man's compromised immune system and exposure to rat droppings could have caused this singular case, Adalja said.

Just because this is the first case of a rat hepatitis E infection documented in a human, Adalja said, "it doesn't mean it's the first time its ever occurred in history." There are many infections that go "undiagnosed or misdiagnosed" he added.

Hepatitis E in rats has a different genetic signature than the human strain, so it's probably not something that routine testing can detect, Adalja said. But because the rat strain is present in rats all around the world, it may be time to modify the diagnostic tests used for hepatitis to include such strains, to see whether this can solve "cases of unexplained hepatitis," he said.

Many people clearly have hepatitis, based on their symptoms, but they test negative on all the human strains known to exist, Adalja said.

The researchers ventured another guess: that the virus's genetic signature could also have changed, giving it the ability to infect humans. But Adalja said he thinks that the man's suppressed immune system is more likely the explanation for this case.

"If this happened in a person who wasn't a liver transplant patient, you can have more basis to speculate" about mutations to the virus, he said. Some next steps in the investigation could be to test people in the same area — especially those who aren't immunocompromised — who may have had rat exposure, he said.

This patient recovered completely after being treated with an antiviral medication that is used to treat the human strain of hepatitis E, according to the Post.

Hepatitis can be caused by various factors, including viruses or alcohol. The human strain of Hepatitis E is typically spread through dirty water, according to the Centers for Disease Control and Prevention (CDC).

The symptoms of hepatitis are typically mild and can include fever, abdominal pain, and jaundice (yellowing of the skin), the CDC says. Most cases resolve on their own, according to the CDC.

Originally published on Live Science.