Moreover, a message from the schools to parents, circulating on social media, warns that the father and sister of the infected patient attended a school father-daughter dance Saturday night at the Ritz-Carlton in Clayton. They also apparently attended a pre-dance gathering at the house of a Villa student.

St. Louis County Executive Sam Page said Sunday that the patient’s family had been told on Thursday to self quarantine at their home in Ladue. Page said the patient’s father had not followed health department instructions. Page spoke at a news conference Sunday evening.

County health officials told the man on Sunday, Page said, “that he must remain in his home or they will issue a formal quarantine that will require him and the rest of his family to stay in their home by the force of law.”