Not everyone is or can be vaccinated, and not everyone vaccinated has immunity. Depending on the virulity of a particular disease, some might be exposed by secondary transmission (e.g., virus particles shed onto an immune individual transferred to a susceptible one). Remember: one (conscious) method of reducing Native American populations was to distribute smallpox-infected blankets among them.

Infants, the very old, and immunocompromised individuals in particular. Given that there's a large population of HIV-infected individuals in the San Francisco Bay Area, that last might be a significant public concern.