The influenza pandemic of 1918 had a major influence on the post World War I world by:

The influenza pandemic had a major influence by killing a lot of people and reducing the average life expectancy

Explanation:

Influenza wave spread for the first time after the end of the first world war. Also known as the Spanish flu, the reported deaths for the first wave was comparatively low.

However in the second wave it resulted in the loss of a lot of lives. Starting from America it later spread to Spain and hence the name Spanish Flu. It killed a lot of people who served in the armed forces for the world war.

preventing European countries from focusing on repairing wartime damage.