Cases of measles and other highly contagious diseases are rising in the United States. Public health experts blame the rise partly on the spatial concentration of parents declining to vaccinate their children, but researchers have given little attention to theorizing why this clustering occurs in particular communities. We argue that residential and school selection processes create “pockets of homogeneity” attracting parents inclined to opt out of vaccines. Structural features of these enclaves reduce the likelihood of harsh criticism for vaccine refusal and foster a false sense of protection from disease, making the choice to opt out seem both safe and socially acceptable. Examination of quantitative data on personal belief exemptions (PBEs) from school-based vaccination requirements in California schools and districts, as well as findings from parent interviews, provide empirical support for the theory. We discuss substantive implications for lawmakers and public health officials, as well as broader sociological contributions concerning neighborhood effects and residential sorting.