Just as Beijing looked poised to claim victory over the coronavirus, a 52-year-old man with fever and chills showed how the pandemic can come roaring back from apparent obscurity.

The patient had shopped for meat and fish at a wholesale market in southern Beijing eight days before testing positive on June 11. His infection marked the Chinese capital’s first reported COVID-19 case in 55 days and scuppered hopes that months of physical distancing, meticulous testing and quarantining had driven the pathogen to extinction in the city of over 20 million.

Now, more than 200 people have tested positive across Beijing, schools are shut and thousands of domestic flights canceled. The resurgence offers a stark warning to countries that appear to have cut chains of transmission: The coronavirus’s ability to cause little or no symptoms in a large proportion of people enables it to spread silently for weeks — even months — creating viral reservoirs that can remain hidden until someone becomes sick enough to warrant testing.