Japan is in the midst of a rubella outbreak that has already infected over 5,000 people in just the first four months of this year. Since the early 2000s, the country has undergone cyclical five-year rubella epidemics, with community-wide outbreaks cresting in the spring and summer. But in the past two years the number of infections has surged dramatically from a hundred-odd cases every year into the thousands, and a weird epidemiological pattern has emerged thanks to a quirk in Japan’s vaccination policy in the 1970s: 77% of cases in the rubella outbreak have occurred in men over the age of 20 (1).