The flu pandemic of 1918 is called the Spanish Flu because the Spanish media were the ones reporting it. Coverage of the flu was censored elsewhere, and the Spanish Flu was likely to have started in the United States.
It had a high mortality rate and its victims were usually between the ages of 20 and 40. It also spread quickly, infecting 1/5 of the world’s population. People died from it died very quickly.
The battlefield conditions of WW1 were ideal for the spread of this flu.
The close proximity of soldiers along with the confining nature of trench warfare allowed the spread of the Spanish Flu among beleaguered soldiers. A side effect of war is disease, and the “mass movements of men in armies and aboard ships probably aided in its rapid diffusion.”