SEOUL — The North Korean leader’s younger sister claims to know the origin of the Covid virus that spread through the country last spring, and she’s threatening vengeance.

South Korea, Kim Yo-jong said, is to blame for enabling balloon launches that along with propaganda messages bring the deadly virus to the North. “Our countermeasure,” Pyongyang’s Korean Central News agency quoted her as saying, “must be a deadly retaliatory one.”

She credited her brother, Kim Jong-un, with masterminding a campaign to stifle the virus while “battling a fever” — the word the North uses for the disease in the absence of adequate facilities for diagnosing it. A video of her remarks was released by Pyongyang. 

Her brother, she said, “could not rest because he was worried about the people.” So moving was that news, according to the video, that “thousands of officials had teary eyes, many women openly wept.”

Neither Ms. Kim nor her brother was quoted in the news agency dispatches in English as saying he’d been ill, but she was reported saying that “he personally came up with packages of various measures and ways, and visited anti-epidemic theaters day and night to teach clear-cut methods … during the 91 days-long arduous campaign reminiscent of a war.”

Ms. Kim spoke right after her brother, general secretary of the ruling Workers’ Party, affirmed at a meeting convened by the party’s central  committee “the priceless victory” gained by “our party’s anti-epidemic policy,” “our state’s counter-crisis strategy,” “the tenacity and the single-minded unity peculiar to our people,” and the “Korean-style socialist system.”

There was no evidence, to be sure, that North Korea had achieved any such victory over a disease that is a menace in South Korea, China, and nearby Japan. North Korean clinics, often bereft of electricity and medicine, are largely powerless to combat Covid-19. 

The North until May was denying the existence of any cases at all among its 25 million citizens. Now it is boasting what would, if true, be the greatest success story of any country in the world in combating the disease — 74 deaths among 4.8 million stricken by the “fever.”

With no foreign aid specialists in the country to assess what’s going on, Mr. Kim credited “the workers in the anti-epidemic and public health sectors across the country and army medics for having dedicated all their strength, wisdom and sincerity to the hard and active work for bringing earlier the victory in the maximum emergency anti-epidemic campaign.”

Kim Yo-jong, after heaping praise on her brother, seemed more interested in inveighing against the leaflet campaign waged by defectors from North Korea.

“Recent national hardship sustained by us was definitely attributable to the hysteric farce kicked off by the enemy to escalate the confrontation with our Republic with the global health crisis as a momentum,” she’s quoted by KCNA as saying.

“If the enemy persists in such dangerous deeds as fomenting the inroads of virus into our Republic,” she went on, “we will respond by not only exterminating the virus but also wiping out the south Korean authorities.”

KCNA, which always refers to “south Korea” with a lower-case “s,” quoted her as saying it was “a matter of grave concern that the disgusting ones in south Korea stage a farce of scattering leaflets, bank notes, awful booklets and things over our territory.”

Her remarks, if nothing else, illustrated the extreme sensitivity in the North Korean hierarchy to the balloon drops — and also testified to their effectiveness.

“We can no longer overlook the uninterrupted influx of rubbish from south Korea,” she said. The fact that it was first reported near the line between the two Koreas “pushed us to suspect the despicable ones in south Korea,” she explained. “It is quite natural for us to consider strange objects as vehicles of the malignant pandemic disease.”

South Korea, under a law enacted while the liberal Moon Jae-in was president, forbids defectors from launching the balloons over the North, but authorities have been looking the other way since the inauguration of Mr. Moon’s successor, the conservative Yoon Suk-yeol, in May.

The unification ministry, responsible for South Korea’s dealings with the North, said Kim Yo-jong’s claims were not only “groundless” but “immensely rude and threatening.”