Whenever I get seasonal allergies, as now, I'm reminded of the fact that this apparently results from our immune systems' being set at too high a level given our actual (these days, in economically advanced societies) routine exposure to internal "enemies." In particular, some of the parasites we regularly hosted in real evolutionary time had apparently evolved ways of tamping down our immune systems, to which we responded (arm's race style) by gearing them up higher. Remove the parasites and their tamp-down effects, and we're left with over-excited immune systems that are roving the body restlessly, determined to root out enemies even if they aren't there.
Small surprise, then, that they invent fake enemies (such as pollen) when they can't find enough real ones, and that this in turn triggers broader bodily disease-fighting responses, even if there isn't actually a disease to fight.
The McCarthyism parallel is getting ancient by now; I suppose one could update it with a reference to presumptive nominees' supposed anti-domestic terrorism policies.