Its a queer thing about that 'flu epidemic in that the main victims were not the usual categories of the elderly and very young, but instead were mainly fit young men in their early twenties - often those living in barracks, rather than in the trenches. The ironic thing is that in many cases those affected were better fed, and physically fitter than at any time in their previous lives.
I don't think its ever been explained - except in that during the war you had large numbers of people living in closer proximity than normal. Its also been suggested that that 'flu was a bit more than normal in terms of disease - a double whammy of a virus and a secondary infection, maybe pneumonia, maybe a fungus, but I don't think the case has ever been proved. Not enough surviving tissue samples to be able to check.