An 11th man tested positive last week for a rat strain of hepatitis E, since it was discovered in 2018 that rats are able to infect humans, but doctors have yet to discover how.The outbreak began in Hong Kong, according to CNN, when a 56-year-old man had post-transplant "abnormal liver functions with no obvious cause." Hepatitis E attacks the liver and causes a fever, jaundice and makes the vital organ increase in size. Although he displayed such symptoms, the human strain of the illness was not found in his system. That is when researchers re-developed a broader test which came back with results showing that he has the rat strain of hepatitis E.The building where he lived was later found to have a rat infestation.Such infections began popping up repeatedly in Hong Kong. The treatment for the human strain on those infected had mixed results, with the last case having been found at the end of April in a 61-year-old man, according to CNN. He particularly baffled scientists, since he had no clear contact with rats.Because people are not testing for this strain of the virus internationally, there could be many more cases that are simply off the radar at this time. Symptoms are often too mild to cause concern. One man was found to have the same strain after a visit to Africa in 2019.While the human strain of the virus is typically transmitted through "fecal contamination of drinking water," researchers have yet to find the reason for the transmission of the rat strain.Dr. Siddharth Sridhar, a microbiologist who was part of the team that made the discovery, said that there may be hundreds cases that have gone undiagnosed.Scientists have taken to testing street rats for the virus, which allows them to understand where certain clusters of rats with the illness lie.Because they do not know how the virus is transmitted, however, it has proven difficult to understand the nature of the strain, as numerous people infected with the virus live among smaller populations of rats.The option of eradicating the rats in the city altogether has proven to be impossible, as the change would be too large and harsh, CNN reports.Much like what is currently speculated about the rampant COVID-19, hepatitis E is a particular danger to those with compromised immune systems or for otherwise vulnerable populations, while the young and healthy are generally not at risk."We need ongoing vigilance in the public to control this unusual infection," Sridhar concluded, according to CNN. "I really hope that public health authorities take the first step and look at how much their populations are actually being exposed to rat hepatitis E."