Why would one child getting measles trigger a city-wide health alert?

Vulnerable groups. Recent cases in Houston and Ohio have been traced to cases brought in by international travelers. Unvaccinated infants and some that had only one dose are most vulnerable to the disease, which can produce death or disability. The surge in vaccine deniers makes the possibility of community outbreaks more likely.
Measles severity. Measles is a very contagious and serious illness that can cause death because of complications like pneumonia and encephalitis (swelling of the brain). In order to prevent further spread of the disease, cases are quarantined until the period of communicability runs out.