It's a cliche that the kids from the rich side of town, who didn't put dirt in their mouths growing up, are the ones who get mono in college. This is true but

It's a cliche that the kids from the rich side of town, who didn't put dirt in their mouths growing up, are the ones who get mono in college. This is true but not necessarily for the reasons you think. Epstein-Barr is largely harmless in younger children, showing up largely as a common cold. It's not that the "farm kids" didn't get infected; it's just that they were infected long before they got to college. (And to preempt a common question: No, we shouldn't proactively expose kids to Epstein-Barr when they're young. While they wouldn't get mono, Epstein-Barr has other long term effects; for example, it has been implicated as a trigger for autoimmune disorders.)