This paper was revised on October 4, 2018
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Females live longer than males in most parts of the world today. Among OECD nations in recent years, the difference in life expectancy at birth is around four to six years (seven in Japan). But have women always lived so much longer than men? They have not. We ask when and why the female advantage emerged. We show that reductions in maternal mortality and fertility are not the reasons. Rather, we argue that the sharp reduction in infectious disease in the early twentieth century played a role. The primary reason is that those who survive most infectious diseases carry a health burden that affects organs, such as the heart, as well as impacting general well-being. We use new data from Massachusetts containing information on causes of death from 1887 to show that infectious diseases disproportionately affected females between the ages of 5 and 25. Both males and females lived longer as the burden of infectious disease fell, but women were more greatly impacted. Our explanation does not tell us why women live longer than men, but it does help understand the timing of their relative increase.