Before going to bed, George Gao, the head of the Chinese Center for Disease Control and Prevention, typically completes his 10,000 steps for the day and then checks the news online. When he scanned his feed on Dec. 30, he was stunned. Two leaked local-government notices warned about cases of unexplained pneumonia in the Chinese megacity of Wuhan. It was the first he’d heard of the outbreak, according to people close to him.

The 58-year-old virologist called the head of Wuhan’s disease-control office, who confirmed the outbreak and revealed to Dr. Gao’s growing alarm that it had been going on since at least Dec. 1, with 25 suspected cases so far, the people close to him said. This wasn’t how things were supposed to work.

Dr. Gao’s agency, known as the China CDC, was set up in 2002 precisely to detect and stop epidemics that often emerge from southern China. It was a mission that grew more urgent after a deadly outbreak that year of severe acute respiratory syndrome, or SARS.

The China CDC trained hundreds of its staff in outbreak-response techniques with U.S. help, sent teams to fight Ebola in Africa and introduced a China-wide, real-time reporting system for infectious diseases. In Dr. Gao, they recruited an expert with credentials from Oxford and Harvard universities.

“I can very confidently say there won’t be another ‘SARS incident,’ ” Dr. Gao said in a speech last year. “Because our country’s infectious-disease surveillance network is very well-established, when a virus comes, we can stop it.”

When the virus did come, though, Dr. Gao’s agency failed at the outset.Instead of the China CDC spotting the outbreak in early December and leading a coordinated response, the virus was rampant in Wuhan by the time Dr. Gao learned of it. By Jan. 23—when authorities ordered the city locked down—it was spreading across the world.

The pathogen would go on to infect more than 21 million people by mid-August, kill more than 760,000, cause trillions of dollars of economic damage and plunge China-U.S. relations into a crisis redolent of the Cold War.