The influenza pandemic, also known as the Spanish Flu, which hit the World the 1918 and lasted until 1920, was the deadliest outbreak of disease in the 20th century. The aim of this study is to analyse the impact that the pandemic had on the belligerent nations of the First World War and, in particular, their armies. How were the armies affected, what impact did the pandemic have on the ability to fight, and what measures did the armies take to contain the virus? Last, but not least, the study will offer some possible consequences and lessons for the British Army of today. Considering that CHACR supports the British Army, it is fitting that the emphasis lies on the UK experience of the Spanish Flu; however, other armies and nations are also taken into consideration to present a more all-encompassing picture of the situation and challenges that the Spanish Flu presented at the end of the First World War.