As more U.S. states reopen and people return to public life, dance fitness classes in South Korea tell a cautionary tale.

A workshop to train instructors for the classes, which are similar to Zumba, ultimately led to more than 100 people falling sick with COVID-19, a new study finds. Nearly 30 teachers participated in the February 15 workshop, which involved intense physical activity for four hours. Later, it was revealed that eight of the participants were infected with the coronavirus, though none had symptoms at the time. By March 9, scientists had identified 112 cases linked to roughly hour-long dance classes at 12 sports facilities and traced them back to the workshop.

The turbulent air flow caused by intense physical exercise in densely populated sports facilities could help spread the virus, the researchers report in the August Emerging Infectious Diseases.

Curiously, one infected instructor taught Pilates and yoga classes of about eight people, but none of those students tested positive for the virus. Those classes’ lower aerobic intensity may be why the virus didn’t spread as easily, the researchers speculate. Class size may also be a factor.

Some states are letting gyms reopen, but there are lots of unanswered questions about whether that’s safe, as detailed by female powerlifter Casey Johnston in Vice.

This article was originally posted on Indoor, high-intensity fitness classes may help spread the coronavirus