In Dae Jang geum, little JG uses her mother's methods of cleaning after a yellow dust storm, and people don't get sick where she is, altho they do everywhere else. It's a clue about germs (or at least spores or something), but how did the mother know, when it wasn't already policy in the royal kitchen?
I think the english subtitles may be inaptly put it was 'germs'. If I remember right, when that episode was shown over here in Singapore cable tv, it used the word 'poison'. The sword used was already swathed with poison.