Final answer:
The influenza that caused the 1918-1919 pandemic, commonly known as the Spanish Flu, killed a disproportionate number of young and healthy adults. The H1N1 strain of the virus was particularly virulent and caused severe pneumonia, leading to high mortality rates.
Step-by-step explanation:
The influenza that caused the 1918-1919 pandemic, commonly known as the Spanish Flu, was unusual in that it killed a disproportionate number of young and healthy adults, rather than the usual victims of infants, the elderly, and individuals with weakened immune systems. The H1N1 strain of the virus was particularly virulent and caused severe pneumonia, leading to high mortality rates. For example, in the United States alone, over half a million people died from the Spanish Flu.