That's vastly more likely to be adaptive than evolutionary.
Evolution isn't even close to the entire expressive picture; there are many non-genetic adaptive factors and even spontaneous mutation.
It's really better to think of a human organism as an adaptive colony, shaped by its distant past but responsive to its environment, even multigenerationally. Just as a little pocket example, we all live with staphylococcus all over our skin, and very often colonizing our noses. When things are in balance, a bacterial diversity and healthy immune response leads to the inability of any one particular bacterium to create a systemic problem. In fact, we have some "good" bacteria such as c. difficile that serve a normal function in the gut that will overcolonize without the environmental pressure of other bacteria and cause significant or even severe health outcomes.
Ultimately, even things like vector-borne illnesses from mosquitos, ticks and parasites can act as environmental or evolutionary imperatives to trigger adaptation or pressures that lead to selection.
Pop-culture "natural selection" assumes a spherical universe in a vacuum; it's an excellent starting point for mapping the relationship between a species and related or ancestral species, but once humans evolved to a societal level, different pressures applied. You actually can observe that in non-human contexts, as well.