Huh? Why would that make a difference? Why would someone even ask such a question?
Bear with me. This all goes back to an article in The British Journal of Homeopathy, Volume 29, published in 1872. It included the following observation:
“Hay fever is said to be an aristocratic disease, and there can be no doubt that, if it is not almost wholly confined to the upper classes of society, it is rarely, if ever, met with but among the educated.”
The reason the upper crust in Merrie Olde England were more prone to having hay fever, along with a host of other ills, is that they weren’t exposed to as many bugs and viruses as afflicted the common weal and therefore had built up little immunity. The rich typically came from smaller families, which also limited their exposure to germs introduced by siblings. Remember, the riff-raff had a much lower standard of hygiene. The rich even washed their hands from time to time! All of which led to a weaker immune system among the upper crust.
Now, think of the hero or heroine in your current work in progress, but do so in terms of the human immune system. The more germs, microbes, and viruses they encounter, the stronger their immune system will be, assuming said germs don’t kill them. Likewise, the more adversity they face, and the more foes they encounter, the more likely they’ll be to survive the ultimate crisis. It’s as simple as that.
If you constantly hose your character down with the fictional equivalent of antibacterial soap, he or she won’t stand a chance when the fertilizer hits the proverbial mixmaster. It would be like doing your children’s math homework for them. They’ll look great right up until they have to perform an equation in class, most likely on a test that’ll mangle their grade point average, and crush your dream of having a successful-looking kid, like a car smasher at a junkyard.
A hundred-plus years after the article cited above, another popped up on the same subject. This one focused on “Hay Fever, Hygiene, and Household Size.” The study examined the incidence of hay fever among 17,414 kids born in the spring of 1958.
Of 16 variables explored, the “most striking” was a comparison between a child’s likelihood of developing hay fever and the number of his or her siblings. It was an inverse proportion; the more brothers and sisters a child had, the less likely he or she was to get the allergy.
Simply put, those extra siblings provided more exposure. Does that mean your hero must constantly battle family members? Maybe, if the setting of your story features a single household. In stories with a slightly larger scope, those siblings are symbolic of the stumbling blocks you must provide for your protagonist in their quest to reach a goal.
For immune systems, it’s probably best if the ongoing exposure doesn’t escalate, though in nature, there’s certainly no guarantee of that. In fiction, however, the opposite should be the norm. The threat level and/or the degree of difficulty should constantly be on the rise. Problems should only get harder, and the villains more despicable as the tale progresses. If you’re unable to find suitable bad guys, there’s always nature, government, and the shortcomings and foibles of your hero to focus on. The point is, things should only get harder, the prize more worthy, and the perils more dire. That’s the way to grow a hero.
Now, lay off the anti-microbial, anti-septic, anti-germ hand soaps. Sneeze once in a while. It might be good for ya!