Disease trackers are calling a choir practice in Washington state a superspreader event that illustrates how easily the coronavirus can pass from person to person. 

The act of singing may have spread the virus in the air and onto surfaces, according to a report from Skagit County Public Health published Tuesday. 

"One individual present felt ill, not knowing what they had, and ended up infecting 52 other people," said lead author Lea Hamner, calling the outbreak a tragedy. 

Two choir members died of COVID-19 after attending the March 10 practice of the Skagit Valley Chorale. The rehearsal was held nearly two weeks before the state's stay-at-home order.

The singers sat six to ten inches apart in different configurations during the two and half hours' rehearsal at a church in Mount Vernon, Washington, about 60 miles north of Seattle, according to the report published by the Centers for Disease Control and Prevention. 

Choir members had no physical contact, although some snacked on cookies and oranges or helped stack chairs, they told investigators. The virus could have spread when exhaled droplets landed on those items. 

Another theory? A fine mist of virus particles emitted during singing could have contributed, the report suggests. Some people emit more particles than others and such emissions can happen with loud talking or, possibly, singing. 

The virus is thought to primarily spread through droplets when an infected person coughs or sneezes. 

The singers felt their first symptoms — cough, fever, muscle pain or headaches — one to 12 days after the practice. The sick singers' average age was 69 and most were women, nearly matching the demographics of choir practice attendees. 

Understanding how the coronavirus spreads is important for preventing and tracking the disease it causes. The CDC recommends avoiding large groups, wearing cloth masks in public and staying at least 6 feet apart from others.

Other known superspreader events include a Chicago cluster of 16 cases, including three deaths, stemmed from a funeral and a birthday party. South Korea is investigating an outbreak linked to nightclubs reopening earlier this month.