פירוש על עבודה זרה 69:8

According to R. Joshua ben Levi, the rabbis prohibited Gentile cheese because Gentiles do not make sure that snakes do not bite their cheese and thereby emit their poison. In other words, Gentile cheese may be poisonous! But we do not tell people that this is the reason for the prohibition because it’s a new prohibition and people may not treat it with all due severity. [One wonders if people would have believed this. If Gentile cheese was frequently enough poisonous, wouldn’t they stop eating it?]