begin quote from:

https://www.cdc.gov/flu/pandemic-resources/1918-commemoration/three-waves.htm

1918 Pandemic Influenza: Three Waves

The 1918 influenza pandemic occurred in three waves and was the most severe pandemic in history.

Graphic: The 1918 influenza pandemic occurred in three waves and was the most severe pandemic in recent history.
Graphic: first wave - spring 1918
Soldiers resting at Camp Funston in Fort Riley, Kansas.
The first outbreak of flu-like illnesses was detected in the U.S. in March, with more than 100 cases reported at Camp Funston in Fort Riley, Kansas.
Graphic: globe showing worldwide spread of flu
During 1918, the U.S. was engaged in WWI. Hundreds and thousands of U.S. soldiers traveled across the Atlantic to deploy for war. The mass troop movement contributed to the global spread of flu.
More people died during the 1918 pandemic than the total number of military and civilian deaths that resulted from World War I.

Graphic: Second wave - fall 1918
Caduceus symbol
In 1918, many health professionals served in the U. S. military during WWI, resulting in shortages of medical personnel around the U.S. The economy suffered as businesses and factories were forced to close due to sickness amongst workers.
Graph: 3 different waves of influenza
There were 3 different waves of illness during the pandemic, starting in March 1918 and subsiding by summer of  1919. The pandemic peaked in the U.S. during the second wave, in the fall of 1918. This highly fatal second wave was responsible for most of the U.S. deaths attributed to the pandemic.

Graphic: Third wave - winter 1918
The Motor Corps of St. Louis chapter of the American Red Cross on ambulance duty during the influenza epidemic, October 1918.
The Motor Corps of St. Louis chapter of the American Red Cross on ambulance duty during the influenza epidemic, October 1918.
A third wave of illness occurred during the winter and spring of 1919, adding to the pandemic death toll. The third wave of the pandemic subsided during the summer of 1919.
An estimated 1/3 of the world’s population was infected with the 1918 flu virus – resulting in at least 50 million deaths worldwide.