And Dr. Siddharth Sridhar, a clinical assistant professor in the university’s department of microbiology, said it suggested that Hong Kong needed to work harder on rodent control, as the city did during the SARS epidemic of 2003 and 2004.

In densely populated areas like Hong Kong, “infections that jump from animals to humans must be taken very seriously,” Dr. Sridhar said. “For these kinds of rare infections, unusual infections, even one case is enough to make public health authorities and researchers very alert about the implications of the disease. One is all it takes.”

Although the patient had received a liver transplant, researchers ruled out infection from his blood and organ donors, which all tested negative for the disease. The researchers instead highlighted rat droppings, piles of uncovered garbage and open passageways found near the patient’s home as major risk factors in rodent-borne diseases.

The researchers said that routine hepatitis E testing would have failed to identify the strain, which is significantly different from the one that typically infects humans. They detected the source of the patient’s infection in this case after finding similarities with infected rats in Vietnam.

When they tested rodent samples that health officials had collected in his neighborhood in recent years, they found hepatitis E in at least one rat.