(CNN) Until last week, Beijing seemed to have all but moved on from the coronavirus pandemic.
For 55 days, the Chinese capital had not reported any locally transmitted infections and life had been returning to normal. Businesses and schools reopened, people went back to work, and the city's public transports and parks were once again teeming with crowds.
But that facade of normality was shattered last week, when a fresh cluster of coronavirus cases emerged from a sprawling wholesale food market in the city, infecting more than 180 people as of Friday.
Within a matter of days, the metropolis of more than 20 million people was placed under a partial lockdown. Authorities reintroduced restrictive measures used earlier to fight the initial wave of infections, sealing off residential neighborhoods, closing schools and barring hundreds of thousands of people deemed at risk of contracting the virus from leaving the city. Some 356,000 people have been tested in just five days.
The flare-up of infections in Beijing, the seat of Communist Party power and previously considered among the country's safest cities, is a stark reminder of how easily the virus can come back to haunt places where it was thought to have been tamed.
Five days before the onset of the current outbreak, Beijing authorities had just downgraded the city's four-tier public health emergency response alert level from Level 2 to Level 3. It was raised back to Level 2 on Tuesday night.
Similar cautionary tales have occurred repeatedly in recent months, with governments rushing to contain reemerging outbreaks after having seemingly brought initial infection numbers under control.
South Korea, much hailed for its success in containing the virus, has been fighting a spike in infections since late May after the easing of social distancing rules and the reopening of schools. Singapore had been considered a coronavirus success story until a wave of infections broke out in April among migrant workers living in packed dormitories.
In China, the initial wave of infections was largely contained by late March, largely thanks to sweeping lockdown measures that brought much of the country to a halt. As outbreaks worsened in other countries, China closed its borders to most foreigners, imposed strict screening at airports and placed all returning Chinese citizens under quarantine. Despite the preventive measures, clusters of local infections still flared up in the country's northeast in April and May, all linked to imported cases.
But the current outbreak in Beijing is the worst resurgence of the coronavirus yet, and authorities are still trying to track down its source.
Previously, reports had linked the outbreak to seafood or meat, after traces of the virus were reportedly detected on a chopping board used by a seller of imported salmon at the market. However, there are now concerns that the virus had been quietly spreading for weeks before it was first detected.
"This outbreak in Beijing probably did not start in late May or early June, but probably a month earlier," said Gao Fu, director of China's Centers for Disease Control and Prevention (CDC), at a meeting in Shanghai on Tuesday.
"There must have been a lot of asymptomatic or mild cases in (the market), that's why the virus has been detected so much in the environment," he said.
Evidence from the United States suggests between 25% to 45% of infected people likely don't have symptoms, with epidemiological studies showing that those individuals can transmit the virus to someone who is uninfected.
Zhang Yong, another Chinese CDC official, agreed with Gao's assessment. He said in an article published by the government Friday that large amount of environmental samples collected from the Xinfadi market had been tested positive for the coronavirus, which showed that "the virus has entered (the market) for quite a while."
"According to preliminary genomic and epidemiological study results, the virus is from Europe, but it is different from the virus currently spreading in Europe. It's older than the virus currently spreading in Europe," he wrote.
Zhang said the virus could have been hidden in imported frozen food products, or lurking in some dark and humid environment such as the Xinfadi market, before it was exposed to people and caused infection.
Over the past months, some Chinese health experts had warned against a potential second wave of infections, even as Chinese state media repeatedly touted the government's success in containing the outbreak and contrasting it with the failures of Western governments.
In an exclusive interview with CNN in May, China's top respiratory expert Dr. Zhong Nanshan warned that China still faced the "big challenge" of a potential return of the virus, and that authorities should not be complacent.
"The majority of ... Chinese at the moment are still susceptible of the Covid-19 infection, because (of) a lack of immunity," Zhong said. "We are facing (a) big challenge, it's not better than the foreign countries I think at the moment."
The outbreak in Beijing will be the latest test of China's coronavirus containment strategy.
On Thursday, Wu Zunyou, chief epidemiologist at China's CDC, struck a victorious tone, declaring that the outbreak in Beijing is already "under control."
Wu said that it is still likely that there will be newly confirmed cases linked to the market emerging in the coming days -- but it is not likely due to fresh transmission.
"Newly diagnosed cases reported every day does not equal new infections and the outbreak being under control doesn't mean there will be zero new cases tomorrow," Wu said.
"There will be cases reported tomorrow and the day after tomorrow. These reported cases are the detection process of the previous infections. Not new infections. The new infections are only sporadic," said Wu.
The chief epidemiologist said that it was not unexpected to see a new outbreak in Beijing, given the large number of global new cases.
"As long as there are risks of imported cases, imported infections and small-scale clusters caused by imported infections might occur anywhere in China. From this point of view, (the Beijing outbreak) is normal," he said.