TO THE EDITOR—The journal’s recent supplement on sex differences in susceptibility and response to infectious diseases was an excellent initiative for promoting research on a neglected topic of major interest. If, in general, males show a higher susceptibility to many infectious diseases, the reviews displayed a number of infectious and autoimmune diseases for which females are more vulnerable. Differential vulnerability between males and females may come from exposure, infection (local or systemic), immune reaction, or a combination of these factors. Evidence came mainly from medicine, epidemiology (direct observation), and biology (animal models and in vivo observation). I address another dimension: demographic evidence.