Almost forgot -- the garter reference in the song. In the film "She Wore A Yellow Ribbon" a wounded trooper (Corporal Quade, as I remember) is having an arrow cut out of his shoulder and, having been liberally dosed with whiskey, breaks into the "soldier's version" of the title tune: "Round her leg she wore a yellow garter..." While this version would not be untypical of parodies sung by soldiers, I believe it was also an invention of John Ford or of one of the writers of the film.

I also checked my on-line version of the Encyclopedia Britannica for "yellow flag" and came up with this: "All over the world a yellow flag is the signal of infectious illness. A ship hoists it to denote that there are some on board suffering from yellow fever, cholera, or some such infectious malady; and it remains hoisted until the ship has passed quarantine. This flag is also hoisted on quarantine stations." Hope this helps.