This article originally appeared on VICE US.

Since patients in a Chinese city started coming down with pneumonia-like symptoms from a mysterious new virus last month, health officials have worried about the potential for a SARS-like outbreak.

Now, new cases have been confirmed in Japan and Thailand, as evidence increasingly suggests that the little-understood virus could be spread by human-to-human transmission.

Japan’s health ministry revealed Thursday that a man in his thirties had tested positive for the new coronavirus strain after his return from the Chinese city of Wuhan, the epicenter of the outbreak. Japanese health officials said the man, a Chinese national, had since been discharged from hospital, and no other cases have been confirmed in Japan.

The case is the second confirmed outside China’s borders, after Thai authorities said Monday that a 61-year-old woman visiting from Wuhan had been quarantined with the virus.

Experts don’t totally understand how the virus spreads, but neither new patient had visited the seafood market in the central Chinese city that’s been identified as the original source of the outbreak. This raises the likelihood that they were infected through human transmission of the virus. Health officials say that while the virus doesn’t appear to transfer easily from person to person, they can’t rule out the possibility that it can be spread through human transmission — a crucial factor in whether it could potentially result in a wider outbreak.

“The fact that some cases do not seem to be linked with the Huanan seafood market means we cannot exclude the possibility of limited human-to-human transmission,” the World Health Organization said in a statement Thursday.

“It is not surprising that there are cases outside of China, and it is possible that there will be cases in other countries in the future.”

The virus, never seen before in humans, first appeared late last month in Wuhan, where one person has died and more than 40 people have been hospitalized, some of them in critical condition, with symptoms including high fevers, invasive lung lesions, and difficulty breathing.

With the mysterious virus bringing back memories of Severe Acute Respiratory Syndrome (SARS), which spread from southern China in 2003 and to killed 774 people, governments across the region have gone on alert, ramping up health screenings at airports. Fears of a wider outbreak have been amplified by the impending Lunar New Year, which begins next week. Hundreds of millions of people travel for the holiday, in one of the world’s biggest human migrations.

Last week, Chinese researchers said they had identified the microbe behind the outbreak as a new strain of coronavirus, and published a draft genome for virologists around the world to analyse, with the hope of better understanding the virus.

Coronaviruses are a large family of viruses that cause illnesses ranging from the common cold to more serious and sometimes deadly diseases, such as SARS, or Middle East Respiratory Syndrome (MERS), which killed hundreds of people in a 2012 outbreak.

Most of the cases in Wuhan have been linked to a local market selling fish alongside live birds, snakes, and rabbits, leading to speculation that the virus, like SARS and MERS before it, may have been transmitted from animals. Samples taken from the market tested positive for the virus, and the facility has since been closed for disinfection.

But the Japanese case adds to the growing evidence that the virus can spread from human to human. Japan’s Health Ministry said Thursday it was possible that while in Wuhan, the patient came in close contact with an infected person.

The patient quarantined in Thailand — who has since recovered — had also not visited the market, suggesting she was either infected by an animal at another location in the city, or by a human carrier. Meanwhile, Wuhan health authorities have reported that the wife of a worker at the market, who had no direct contact with the facility, tested positive after her husband was infected.

Despite the increasing likelihood that the virus can be spread between humans, health officials are calling for calm. The WHO said Thursday there was still “no clear evidence of sustained human-to-human transmission,” and no need to avoid trade or travel with Wuhan. U.S. health officials have issued a warning for visitors to Wuhan to “be aware and practice usual precautions.”

Japanese health officials said Thursday that they did not believe they were facing a wider outbreak, and there had been no suspected cases of secondary infection among the patient’s family members or medical staff.

“The chances are slim that the infection will spread [in Japan],” a health ministry official said in a statement.

But the WHO acknowledged that researchers were still largely in the dark about the new virus and the impact it could have. “We are still in the early stages of understanding this new virus, where it came from, and how it affects people. There [are] still many unknowns, and the situation may continue to evolve.”