According to your linked article, the Spanish Flu took a heavy toll on the young and healthy specifically because it caused immune systems to overreact, which meant that healthy people with healthy immune systems were WORSE HIT since their "healthy" immune systems overreacted more, which was a greater risk to the person.
I'm not quite sure what points about the immune system one can draw from this, but if I were struggling through the Spanish Flu's time, I'd be trying to get the vaccine as fast as I could and hoping my immune system wouldn't react like a healthy person's to the illness.