1918 "Spanish" Influenza Pandemic
The influenza pandemic that occurred in 1918 is considered one of the worse calamities to have occurred in human history.
This virus was particularly noted for its tendency to kill young people.
from Taubenger and Morens
There were three waves of infection
Chart of waves of death in the UK from Taubenger and Morens
In the Northern Hemisphere, peaks in death occurred at slightly different times in different cities, but primarily in fall and winter.
References
Nicholls (2006) Pandemic Influenza: The Inside Story. PLoS Biology, e50
Taubenberger and Morens (2006) 1918 Influenza: the Mother of All Pandemics. Emerging Infectious Diseases.
Influenza 1918 - PBS