Protective immunity and susceptibility to infectious diseases: lessons from the 1918 influenza pandemic

The influenza pandemic of 1918 killed nearly 50 million people worldwide and was characterized by an atypical W-shaped mortality curve, where adults between the ages of 30–60 years fared better than younger adults aged 18–30 years. In this review, we will discuss why this influenza virus strain was so virulent and how immunological memory to the 1918 virus may have shaped the W mortality curve. We will end on the topic of the 'honeymoon' period of infectious diseases—the clinically documented period between the ages of 4–13 years during which children demonstrate less morbidity and/or mortality to infectious diseases, in general, compared with young adults.