A quickly escalating measles outbreak around Portland, Ore., has led health officials in nearby Clark County, Wash., to declare a public health emergency as they warn that people infected with the highly contagious virus have visited schools and churches, a dentist’s office, a Costco, an Ikea and an Amazon locker pickup station.

Someone with measles was at Concourse D of the Portland International Airport on Jan. 7, the county’s public health department advised. An infected person attended a Portland Trail Blazers home game Jan. 11.

At the beginning of last week, there were only a handful of confirmed cases. On Friday, the day the emergency was declared, there were 19. By Sunday, that number had grown to 21. The latest update came Tuesday, when county officials said they had confirmed 23 cases and were investigating two more suspected cases. The vast majority of those who have fallen ill had not been immunized.

The outbreak makes concrete the fear of pediatric epidemiologists that a citadel of the movement against compulsory vaccination could be susceptible to the rapid spread of a potentially deadly disease.