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The swift spread of coronavirus from a group of dance fitness instructors in South Korea offers a cautionary tale as countries worldwide and parts of the U.S. begin to slowly reopen during the pandemic.

In early February, a group of dance fitness instructors in South Korea met for a workshop. It comprised four hours of intense aerobics with 27 people in a tiny space. At that time, they didn't know that eight participants had coronavirus (but were not showing serious symptoms).

A few weeks later, the virus had infected 112 people across 12 different fitness facilities in the region, all linked to dance fitness classes, according to a report by the Centers for Disease Control and Prevention.

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Contact tracers in South Korea were able to link that outbreak of cases to instructors from the initial February workshop, who were discovered to have worked while experiencing mild symptoms of coronavirus, such as a cough.

A bodybuilding gym in Idlib province northwestern of Syria is sterilized on April, 7, 2020.

A bodybuilding gym in Idlib province northwestern of Syria is sterilized on April, 7, 2020. (Photo by Karam Almasri/NurPhoto via Getty Images)

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About 50 percent of the 112 cases were found to have been transmitted from instructors to students. That's 54 students out of the 214 found to have been exposed in the classes, or an infection rate of 26 percent, the report explained.

On average, students began experiencing symptoms about 3.5 days after attending a dance fitness class, researchers found.

Factors that may have led to increased transmission from the instructors, according to researchers, include "large class sizes, small spaces, and intensity of the workouts. 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."

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As of Tuesday afternoon, there were more than 1.5 million infections and at least 91,187 deaths from COVID-19 in the United States.