A measles outbreak in Philadelphia has expanded to eight confirmed and three suspected cases with three patients hospitalized, city health officials said Monday. Vaccine-preventable diseases such as measles and polio remain a public health threat. And with childhood vaccine hesitancy on the rise, the problem could get worse.

Officials at the Philadelphia Department of Public Health are now warning the public about several known exposure sites across the city, including healthcare facilities and a daycare center.

Placed in a larger context, the current situation in Philadelphia speaks to a wider problem of the return of vaccine-preventable diseases as a result of the steady increase in vaccine hesitancy in recent years.