Auto immune makes sense to me. It’s the first thing that comes to mind when it’s idiopathic. Hard to find the root cause because the patient needs to have a preexisting susceptibility, a depleted immune system, and then some external trigger. To observers it seems random. 

 The correlations between the poisonous gasses used in WWI and the "Spanish influenza" are interesting.. the fact that this also seemed somehow tied to the flu took my interest.