A disease once thought relegated to history is reasserting itself, and the medical community is responding not with alarm but with the sober recognition that prevention requires constant tending. Measles, preventable yet highly contagious, is rising again in populations where its return was once considered unlikely, prompting clinicians and health administrators to revisit the infrastructure of vigilance they had hoped would never be urgently needed. The moment raises a deeper question that haunts all public health progress: whether the victories of one generation can be reliably inherited by