After 15 months leading the world as a case study in epidemic management, Taiwan’s bubble has burst.

In just one week earlier this month, the case count for COVID-19 infections spiked by more than 40%, and it’s likely that figure will keep climbing. Compared to India, which reported 311,170 new cases on May 16 alone, Taiwan’s then-784 number in a week was minuscule. Yet the sudden surge, bringing total cases now to nearly 5,000 since the start of the pandemic, forced authorities to implement unprecedented restrictions and has people wondering what went wrong.

At least some of it can be traced back a month ago to pilots returning from overseas. While Taiwan has imposed a strict 14-day quarantine on arrivals for more than a year, air crews faced just a five-day isolation period. This freedom allowed them to hit the town soon after returning home, as was the case with one pilot who visited bars and a barbeque restaurant six days after a flight from the U.S. He subsequently tested positive. And he wasn’t the first.