A woman coughing at a birthday party with friends and family 'joked' she had coronavirus but later found out she actually did - after infecting others.
The party took place after a stay at home order was issued in Pasadena, California, in March.
But a "large number" of extended family members and friends still attended, health officials said.
One woman at the party was coughing while not wearing a face mask, mixing among other guests who were also not wearing face coverings.
Lisa Derderian, a spokeswoman for Pasadena told CNN : "She was joking with people at the birthday party.
"She said I may have Covid-19, and lo and behold, she did."
"It was selfish behaviour that could have been avoided," she added.
Five other people who attended the party later tested positive for coronavirus.
It is thought another six people could have the disease after showing symptoms, but they are yet to be tested.
A news release by the Pasadena Office of the City Manager said: "Pasadena Public Health Department (PPHD) recently identified a cluster of COVID-19 cases among attendees of a birthday party.
"Through contact tracing, the PPHD disease investigation team discovered more than five laboratory-confirmed COVID-19 cases and many more ill individuals."
The statement continued: "The party was attended by a large number of extended family members and friends after the Safer at Home Order was issued.
"The index case, or the first patient in the outbreak identified with the disease, was coughing and not wearing a face covering at the party.
"Guests were also not wearing face coverings or practicing social distancing.
"As a result, COVID-19 spread among party attendees."
