Long before vaccines reshaped the landscape of childhood illness, measles moved through England and Wales with a quiet, almost calendrical logic. Researchers have now listened carefully to that old rhythm — two decades of case records from 1948 to 1968 — and found within it a signal: the size of a susceptible population at the start of any given year reliably foretells the severity of the outbreak to follow. It is a reminder that epidemics are not random storms but the accumulated consequence of births, recoveries, and the slow arithmetic of immunity — and that history, read carefully enough,