BEIJING >> China is pushing a new theory about the origins of the coronavirus: It is an American disease that might have been introduced by members of the U.S. Army who visited Wuhan in October.

There is not a shred of evidence to support that, but the notion received an official endorsement from China’s Ministry of Foreign Affairs, whose spokesman accused American officials of not coming clean about what they know about the disease.

The intentional spreading of an unfounded conspiracy theory — which recirculated on China’s tightly controlled internet today — punctuated a downward spiral in relations between the two countries that has been fueled by the basest instincts of officials on both sides.

The insinuation came in a series of posts on Twitter by Zhao Lijian, a ministry spokesman who has made good use of the platform, which is blocked in China, to push a newly aggressive, and hawkish, diplomatic strategy. It is most likely intended to deflect attention from China’s own missteps in the early weeks of the epidemic by sowing confusion or, at least, uncertainty at home and abroad.

Zhao’s posts appeared to be a retort to similarly unsubstantiated theories about the origins of the outbreak that have spread in the United States. Senior officials there have called the epidemic the “Wuhan virus,” and at least one senator hinted darkly that the epidemic began with the leak of a Chinese biological weapon.

“The conspiracy theories are a new, low front in what they clearly perceive as a global competition over the narrative of this crisis,” said Julian B. Gewirtz, a scholar at the Weatherhead Center for International Affairs at Harvard.

“There are a few Chinese officials who appear to have gone to the Donald J. Trump School of Diplomacy,” added Gewirtz, who recently published a paper on China’s handling of the AIDS epidemic, after a similar disinformation campaign. “This small cadre of high-volume Chinese officials don’t seem to realize that peddling conspiracy theories is totally self-defeating for China, at a moment when it wants to be seen as a positive contributor around the world.”

The circulation of disinformation is not a new tactic for the Communist Party state. The United States, in particular, is often a foil of Chinese propaganda efforts. Last year, Beijing explicitly accused the U.S. government of supporting public protests in Hong Kong in an effort to weaken the party’s rule.

The old tactic has been amplified by more combative public diplomacy and a new embrace of a social media platform that is blocked in China to spread a message abroad.

Victor Shih, an associate professor at the University of California at San Diego who studies Chinese politics, said that while the campaign was very likely an attempt to distract and deflect blame, a more worrisome possibility was that some officials fabricated the idea and persuaded top leaders to believe it.

“If the leadership really believes in the culpability of the U.S. government,” he warned, “it may behave in a way that dramatically worsens the bilateral relationship.”

China’s leader, Xi Jinping, has faced sharp criticism for the government’s initial handling of the outbreak, even at home. Public anger erupted in February when a doctor who was punished for warning his colleagues about the coronavirus died, prompting censors to redouble their efforts to stifle public criticism.

Chinese officials have repeatedly urged officials in other countries not to politicize what is a public health emergency. Conservatives in the United States, in particular, have latched on to loaded terms that have been criticized for stigmatizing the Chinese people. Secretary of State Mike Pompeo referred to the “Wuhan virus,” while Rep. Kevin McCarthy, R-Calif., called it the “Chinese coronavirus.”

In response, Chinese officials and state news media have stepped up their criticism of American officials’ comments.

Only days before Zhao’s latest post, the Xinhua news agency published a commentary denouncing “Washington’s poisonous coronavirus politics” and warning that spreading rumors simply encouraged “fear, division and hate.”

“Their dangerously irresponsible statements are highly counterproductive at this drastic hour that demands solidarity and cooperation,” the commentary, written by Gao Wencheng, said, “and could be much more menacing than the virus itself.”

The coronavirus, according to all evidence, emanated from Wuhan, China, in late December. Scientists have not yet identified a “patient zero” or a precise source of the virus, though preliminary studies have linked it to a virus in bats that passed through another mammal before infecting humans.

A senior official of China’s National Health Commission, Liang Wannian, said at a briefing in Beijing last month that the likely carrier was a pangolin, an endangered species that is trafficked almost exclusively to China for its meat and for its scales, which are prized for use in traditional medicine.

The first cluster of patients was reported at the Huanan Seafood Wholesale Market, and studies have since suggested that the virus could have been introduced there by someone already infected. Wuhan and the surrounding province of Hubei account for the overwhelming amount of cases and deaths, so there is no scientific reason to believe the virus began elsewhere.

Zhao’s assertion began with a post linking to a video of the director of the U.S. Centers for Disease Control and Prevention, Robert R. Redfield, testifying before the House on Wednesday and suggesting that some flu deaths might have been caused by the coronavirus.

“When did patient zero begin in US?” Zhao wrote on Twitter, first in English and separately in Chinese. “How many people are infected. What are the names of the hospitals? It might be US army who brought the epidemic to Wuhan. Be transparent! Make public your date! US owe us an explanation.”

Zhao appeared to refer to the Military World Games, which were held in Wuhan in October. The Pentagon sent 17 teams with more than 280 athletes and other staff members to the event, well before any reported outbreaks. The Pentagon has had confirmed cases in South Korea and Italy and is bracing for more to emerge, but no illnesses have been tied to American service members from October.

Zhao’s remarks were spread on China’s most prominent social media platform, Weibo, under a hashtag: #ZhaoLijianPostedFiveTweetsinaRowQuestioningAmerica. By late afternoon on Friday, that hashtag had been viewed more than 160 million times, along with screenshots of the original Twitter posts.

The State Department summoned the Chinese ambassador on Friday to protest Zhao’s comments, officials in the Trump administration said.

At the regular briefing at the Ministry of Foreign Affairs on Friday, another spokesman, Geng Shuang, sidestepped three questions about whether Zhao’s suggestion had politicized the crisis and reflected official Chinese policy.

He instead noted the statements made by American officials and lawmakers to “smear and attack” China.

“We are firmly opposed to this,” he said. “In fact, the international community, including the United States at home, have different views on the source of the virus. What I have been saying in recent days is that the Chinese side always believed that this is a scientific issue and requires scientific and professional opinions.”

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