But that's exactly what happened in Pasadena in L.A. County last month. And when a woman infected with coronavirus showed up, and was coughing at the party without a mask, public health officials say the virus took off.
"There was someone who was coughing who attended the party and there were subsequently five laboratory confirmed cases of COVID-19 and many other people who are ill with COVID symptoms after that," said Dr. Ying-Ying Goh, the city of Pasadena's health officer.
Pasadena public health workers were able to track the spread of the virus back to the party by contact tracing. Officials say guests at the party did not wear face coverings or practice social distancing.
Health officials said the party was attended by "a large number" of extended family members and friends after the coronavirus stay-at-home order was issued.
But what's unknown right now is if any of the five infected partygoers have also spread the infection to others over the past few weeks.
A cluster of #COVID19 cases in Pasadena has been traced to a birthday party. Remember, the best way to celebrate #MothersDay and any other occasion is by STAYING HOME and staying healthy: https://t.co/YyAqMebKhF #StayHomeSaveLives pic.twitter.com/WI9uh6zTKs— City of Pasadena (@PasadenaGov) May 9, 2020
"Everyone is asked to stay at home," Goh said. "They're ordered to stay at home, but we're just asking for the public's cooperation to keep yourself and your family safe."
City officials say no citations were issued in connection with the party.