> Why is that "sad"? Nature has gone the same path. We have basic defenses that are "on" all the time (passive immune system - nonspecific), and we have an adaptive response that reacts to what actually happens to us

While it's true that nature has taken the same path for the same reasons, I don't think I'd have to look very hard for people to agree that the fact that people fall ill, sometimes seriously so, is "sad".