"How could gluten, present in a staple food that has sustained humanity for thousands of years, have suddenly become so threatening?"The answer - the obvious, simple, Occam's Razor-sharp answer - is that only mass delusion can do that (same as the Ebola hysteria, where the American death toll's climbed to "still zero"). Mass delusions happen often and are well-studied, yet we still get caught up in them. If you want to understand the craziness crowds of humans are capable of, the definitive work is "Extraordinary Popular Delusions and the Madness of Crowds" (here it is for a little more than a buck in a Kindle version; here's the Wiki rundown). History reveals that human throngs have always had some irrational and massive this-or-that going on.
It seems like everyone's forgotten that we used to swoon, and to succumb to "nervous exhaustion". Only recently, a very large number of people suffered from "chronic fatigue syndrome" and TMJ, but we don't hear much about that stuff anymore, either. With all those conditions and syndromes, there was actual virulence at work, but it was an idea virus. And such viruses can absolutely flair bona-fide physical symptoms ("psychosomatic" is, after all, half somatic!). It's been evident for some time that mass sentiment can foment real malady, just as it can foment courage or hatred. And tummy-aches - endlessly rebranded under a succession of serious-sounding names - have perennially been a go-to complaint for the high-strung.
And, of course, business fans the flames, because there's always profit to be made from a nice fat mass delusion (check out Mcdonalds' response to gluten madness). But it's a risky move, because, like any virus, these things die out as swiftly as they conflate. We'll eventually move on to new ways to account for the fact that idle time - a new development in human life - gives neurotic people the opportunity to pay lots of attention to the fact that they don't feel quite as vibrant as they imagine they're entitled to feel.
One more observation. Mass delusions are stirred up tribally, and anti-gluten craziness is particularly popular on the left. Same for anti-vaccine craziness (I heard a statistic recently - which I haven't been able to confirm - that in parts of northern California vaccine non-compliance among children approaches 75%). It's illuminating to note that adherents of both tend to be highly-educated people (smart people aren't immune to human folly!); the same people who stridently chastise the right for being anti-science. And they're correct: the right is anti-science. But so's the left. Human beings are anti-science.
It's a useful exercise, when observing bad thinking or behavior in others, to pin it not on their most noticable distinguishing characteristic (lazy blacks, greedy jews, chuckle-headed Republicans, bleeding-heart Democrats, etc etc), but on their humanity. As I once wrote:
We study the Other...and we don't like what we see. Men rue the cruelty of women; women rue the cruelty of men. Both are quite correct, really.
Racism, sexism, classism, etc. are nothing more than the incomplete registration of a perfectly appropriate misanthropy.