North Korea has shut its borders to foreign tourists in a bid to keep out the mysterious, pneumonia-like coronavirus spreading in China next door.

A leading tour operator, Young Pioneer Tours (YPT), said its travel partners in North Korea had informed them that they were unable to accept tourists temporarily due to the virus. On its website, it called the move a “precaution” against the virus that has killed nine people and sickened more than 400 in China. There have been no reported cases of suspected infections in North Korea.

The move came as the United States on Tuesday confirmed the first case of the illness within its borders, after a traveler from Wuhan, the epicenter of the outbreak in China, landed in Washington state.

Tourism is one of the few North Korean industries not under international sanctions imposed by the United Nations and Washington, making the sector vital to Pyongyang’s embattled economy. Most tourists enter the politically restricted and reclusive country through the border with China and are Chinese nationals.

North Korea closed its border to tourists for four months during the height of the Ebola epidemic in Africa in 2014-15 and also imposed a 21-day quarantine on diplomats and aid workers entering the country.

“I would assume that North Korea will treat this matter much like Ebola back in 2014-2015 so this will affect all foreigners going to North Korea,” Rowan Beard of YPT said in an email, adding that no quarantine has yet been announced by North Korean customs authorities.

“When the DPRK prevented foreigners from entering North Korea from the Ebola scare this did take an impact on our business and with most of our competitors closing their business in the DPRK,” he added, referring to the country’s official name, the Democratic People’s Republic of Korea. “We’re hoping this time this it won’t last as long as Ebola.”

Two Chinese travelers infected with coronavirus have been reported in Thailand, according to officials there. A man who had traveled from China this month is now sick with the disease in Japan, while a Chinese women entering South Korea on Sunday has also been isolated after being found to be infected. Australia and the Philippines are investigating suspected cases in their countries.

While experts were puzzled by the border closure in 2014 during an Ebola outbreak half way across the world, this week’s move was understandable given the outbreak of a virus in North Korea’s neighbor and largest trading partner.

“The DPRK government’s costly decision to suspend all foreign tourists is a reasonable strategy to minimize the risk of domestic spread of the new virus,” said Kee Park, a global surgery scholar at Harvard Medical School who has worked extensively in North Korea.

“The country’s capacity to detect and treat the viral infection once it enters may not be adequate,” he said. “Therefore, by drastically reducing the entry of potentially infected people the government hopes to mitigate the risk.”

The closure of the border to tourists is also a setback for South Korean President Moon Jae-in’s plans to allow tourists from his country to enter North Korea. Moon proposed the idea in a news conference last week in an attempt to improve frosty relations with the North, but it’s far from clear whether Pyongyang will be interested.

Coronavirus in rare cases can spread from animals to humans, and, in even rarer cases, that same strain can then be transmitted from human to human. Health officials surmise that is what is happening here, though scientists are still investigating how exactly the disease has spread.

Scientists have not identified this strand of coronavirus in humans before, so it does not have a specific name yet.

In 2002, Asia grappled with the outbreak of a similar kind of disease in the same coronavirus family, known as severe acute respiratory syndrome, or SARS. The disease killed 774 people and infected more than 8,000 people before being contained eight months later, in part because of screenings at airports and quarantines of infected people.

North Korea took a similar, but not as serious, step in 2003 when it halted a tourism project with South Korea, citing fear of SARS. It also restricted entry to people from areas affected by the disease.

In 2015, the North’s Korean Central News Agency, known for outlandish claims, announced North Korea had found a cure for SARS and a related virus, MERS, as well as for AIDS and Ebola. It did not provide any evidence.

Boosting tourism has been one of North Korean leader Kim Jong Un’s few avenues for injecting much-needed cash into the economy. He has worked to court Chinese travelers, and the number of Chinese tourists had been rising since‚Äč President Xi Jinping of China visited North Korea in June.

Xi’s visit was also seen as a diplomatic victory for Pyongyang after denuclearization talks with President Donald Trump collapsed in February, raising the specter of heightened instability in the region.