For the first time ever, rat hepatitis has infected a human being.  This case has reignited the long-standing questions surrounding rats, humans, and the hepatitis E virus.

Last Friday, on 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, according to ARS Technica. 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.

The hepatitis E virus spreads through fecal matter (poop). You can also catch it if you drink or eat something that has been in contact with the stool of someone who has the virus. Hepatitis E is more common in parts of the world with poor handwashing habits and lack of clean water, according to WebMD. The virus occurs much less often in the U.S., where water and sewage plants kill it before it gets into the drinking supply. You also can contract hepatitis E if you eat undercooked meat from infected animals, such as pigs or deer. Less often, you can get the virus from raw shellfish that comes from tainted water.

Researchers have documented that human versions of this virus cause outbreaks of acute liver inflammation in developing countries, typically in cases where fecal contamination seeps into water sources. The human viruses cause an estimated 20 million infections a year around the world, leading to tens of thousands of deaths, according to the World Health Organization.

In the case of hepatitis E jumping from a rat to a human, the case revealed by the University of Hong Kong appears to be the first of its kind. 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 made attempts to track down its source.

In the course of their work, the researchers ruled out the possibility that the man was infected from his organ or blood donors. Instead, the researchers noted evidence of a rodent infestation near the man’s home with noticeable collections of droppings by a next-door garbage chute. Testing of at least one rat collected in the neighborhood in recent years had turned up positive for rat hepatitis E, the researchers reported.

Infectious disease expert and microbiologist Yuen Kwok-yung, of the University of Hong Kong, described the case “a wake-up call” in a press conference. He said some of the city’s rats had become larger than cats and urged officials to improve sanitation to avert public health risks from the menacing rodents. –ARS Technica

According to hepatitis E expert X.J. Meng of the VA-MD College of Veterinary Medicine, there is no need to be concerned just yet. “This has never happened before,” Meng noted of the virus jumping from a rat to a human. It’s “unprecedented” he went on to say in an interview with Ars Technica. “Regardless, I do not personally think this [case] is a major public health issue.”  Meng believes the man who became infected with hepatitis E from a rat had a suppressed immune system due to his liver transplant and the body attempting to accept the new organ.

The case does, however, highlight the mystery surrounding the little-known form of the virus. In a 2016 article, Meng noted that humans simply don’t know enough about hepatitis E viruses to figure out what’s could be going on.