The World Shudders as President Trump Tests Positive for Covid-19

“This highlights that what has always been destabilizing about Trump’s administration is not really his policies — it is him,” said Jeremy Shapiro, an Obama administration national security official who is now research director at the European Council on Foreign Relations. “In this sense, the worry from foreign governments will likely be its effect on the president’s fragile psyche.”

Britain’s experience shows that even in a country with a well-organized political system, a leader’s sudden illness can be deeply unsettling. When Mr. Johnson contracted the virus in March, the government was adrift for several days while he struggled to keep leading the response to the pandemic, via Zoom calls, from isolation in his official residence on Downing Street.

When Mr. Johnson, 56, was admitted to the hospital and then to intensive care, he deputized the foreign secretary, Dominic Raab, to act in his absence. But that did little to dispel the uncertainty, especially since unlike in the United States, there is no legal line of succession if a prime minister dies in office or is permanently incapacitated.

The government issued upbeat but unrevealing reports of Mr. Johnson’s health, and after he was released from the hospital on Easter Sunday, he disclosed that his condition had been more grave than was reported.

In Brazil, Mr. Bolsonaro’s bout with the virus was less serious. He said he suffered only mild fever and body aches before testing positive on July 7. After quarantining on the grounds of the presidential residence in Brasília, he pronounced himself recovered on July 25, posting a photo of himself smiling and giving a thumbs up.

Mr. Bolsonaro, 65, who has adopted Mr. Trump’s approach of playing down the virus and promoting miracle cures, appeared to brandish a box of hydroxychloroquine pills, the anti-malaria medicine. Despite claims by Mr. Trump, there is growing scientific consensus that the drug is not effective in treating Covid-19.