The party was held after the city issued stay-at-home orders March 19 and was attended by a large number of extended family members and friends who did not wear face coverings or stay six feet apart, the city said in a news release.

“One person showed up to the party exhibiting symptoms and joking she may have the virus,” Lisa Derderian, spokeswoman for the city of Pasadena, said in an email. “The aftermath affected several others who became seriously ill because of one person’s negligent and selfish behavior.”

Through contact tracing, a Pasadena Public Health Department disease investigation team discovered more than five laboratory-confirmed coronavirus cases among attendees of the party “and many more ill individuals,” the city said. The team identified the woman as the index case, or the first patient in the outbreak identified with the infection, officials said.