The researchers noted that the classes involved many people engaging in high-intensity exercise in a small space — a pretty ideal environment for COVID-19 spread.

"The moist, warm atmosphere in a sports facility coupled with turbulent air flow generated by intense physical exercise can cause more dense transmission of isolated droplets," the authors said. The number of people in a class also appeared to play a role in disease spread: Typical-size classes, with up to 22 participants, resulted in disease spread, while small classes with fewer than five people did not result in any spread of COVID-19, the report found.

Interestingly, one of the infected instructors also taught yoga and pilates, but none of these students became infected. The authors hypothesized that the lower intensity of these activities did not result in the same level of transmission as the high-intensity fitness classes.

Overall, "because of the increased possibility of infection through droplets, vigorous exercise in closely confined spaces should be avoided during the current outbreak," the authors concluded.