At the late September yodelling event in the rural Schwyz canton, people in the audience were asked to maintain social distancing, but mask-wearing was not required.
“We can't do anything about what happened with this yodelling group. We found out nine days after the performances that several people from the group were infected,” event organiser Beat Hegner told RTS public television.
Now the pandemic has spread through the region, with 1,238 cases compared with just 500 in mid-September.
On Wednesday alone, 94 people tested positive, twice as many as the day before.
The overloaded cantonal hospital has asked people to begin wearing masks and avoiding gatherings.
“There's an extremely high rate of positive tests. We've gone from 30 to 50 percent,” hospital chief Franziska Foellmi said.
“It's time we reacted. The explosion in the number of cases in Schwyz is one of the worst in all of Europe,” chief doctor Reto Nueesch posted online.
Cantonal authorities have stepped up infection control measures, making mask-wearing compulsory at all public and private events with more than 50 people and in situations where distancing can't be maintained.
But people can still go to the shops without covering their nose and mouth.
Switzerland isn't the only country to practice yodelling, an age-old style of singing where the performer rapidly switches between registers.
It's also practised in Austria's Tyrol region and in variant forms across the mountains of central Europe, from Poland to Romania.
Like archery, wrestling and the Alpine games, yodelling has been one of the building-blocks of common identity between Switzerland's culturally disparate regions since the 19th century.