Complicated answer. Either because of radiation, poisoning, disease, or antibiotics, the flora might be damaged or even pretty much imbalanced. Or, it might be having a bad interaction with an individual’s immune system.
Traditionally, a way of treating it was with what is called a “fecal transplant”, aka a “chocolate shake”, quite literally, a small amount of a healthy person’s feces put in a chocolate shake for the person with a poor flora to drink.
But this is a primitive way of doing things. Human flora contains some 300-1000 different microorganisms, though only 30-40 take up almost all the available space. And the presence or absence of any of these might be responsible for serious medical problems.
Okay. We have quite a few chocolate candy bars leftover from tonight's trick or treating, which suddenly aren't so appealing anymore.
My dentist and my waistline thank you.