I don't grasp the expression bread-crumb in this phrase : "physicians would discover slowly, leaving behind a bread-crumb trail of reports like this one :....".
Thanks for your explanations!
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"Bread crumbs" are a metaphor for leaving a trail behind. Sometimes this is deliberate, such as to mark where you've been when exploring. Sometimes it's accidental, such as when a thief leaves clues like footprints at the scene of the crime, which detectives can "follow" later to find the thief.
The metaphor may have originated with the German fairy tale Hansel and Gretel in which the characters leave behind bread crumbs so that they can find their way in the woods.
In modern language, "bread crumbs" also refer to a common style of navigation links on a web page or in a computer program, where the links are broken out hierarchically to show the user where they are in the structure of the program like a trail of bread crumbs through the woods.