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.