Would we even notice if we read ancient truths that had yet to be rediscovered? Rerum rusticarum libri III was around in the middle ages, wasn't it? Wouldn't scholastics have read it? It's a "holy crap!" moment after you discover the germ theory of disease. Before that it's a bizarre superstition of antiquity.
This passage is a mention of the germ theory of disease. Cochran suggests that there is a lost manuscript actually discussing it, intended for people who don't already know the theory.
Related: Son of Low Hanging Fruit, Low Hanging Poop
A post by Gregory Cochran's and Henry Harpending's blog West Hunter.