In mid-July, Vietnam still shone as a Covid-19 outlier. No reported deaths, and months without a locally transmitted case.
Fans packed into football stadiums, schools had reopened, and customers returned to their favourite cafes.
"We were already back to normal life," said Mai Xuan Tu, a 27-year-old from Da Nang in central Vietnam.
Like many in the coastal city wildly popular with domestic visitors, she works in the tourism industry and was slowly resuming bookings for the tour company she founded.
But by the end of July, Da Nang was the epicentre of a new coronavirus outbreak, the source of which has stumped scientists. Cases suddenly surged after 99 straight days with no local transmissions.
Scientists and researchers across the country are racing to find out exactly how it did.
In Hanoi Prof Rogier van Doorn, director of the Oxford University Clinical Research Unit, says the source of this latest outbreak remains a "big mystery".
His team works with the government on its infectious diseases programmes and some of them focus on what he calls "genetic detective work" - the sequencing of viruses that can help illuminate "the chains of transmission. Who or where the virus came from".
But so far no-one knows how the first new case in Da Nang - a 57-year-old man known as patient 416 - came into contact with the coronavirus.
