The first known case of rat hepatitis jumping to a human patient has reopened a long-standing mystery of how the cryptic viruses spread and bounce between humans and animal reservoirs.

Last Friday, September 28, researchers at the University of Hong Kong revealed that a 56-year-old-man had contracted a strain of hepatitis E previously thought to only infect rats. Hepatitis E viruses, generally, cause liver inflammation in humans that is usually self-limiting but can become severe or even fatal to some, including organ transplant patients and pregnant women.

Researchers spotted the man’s infection back in September of last year, after he had undergone a liver transplant in May. They reported that the man’s infection was treated and cleared by March of this year, after which they verified the presence of the unexpected virus and tried to track down its source.