The virulence of the national appetite for bogus revelation.

Mencken is condemning what he saw as a distinctly American hunger for sensational, counterfeit “revelations”—claims of divine messages, prophetic certainty, miracle cures, or other forms of credulous certainty marketed as truth. Calling the appetite “national” suggests a mass cultural phenomenon rather than isolated gullibility, and “virulence” frames it as a social disease: contagious, aggressive, and hard to cure. The phrase fits Mencken’s broader critique of American anti-intellectualism and religious or ideological enthusiasm, where emotional gratification and moral drama can outweigh evidence. The line’s sting comes from its compressed irony: the public does not merely tolerate bogus revelation; it craves it.