And all along I thought the 'thieve's blend' of oil and combination of herbs was coined by the thieves themselves. There's no saying whether or not the doctor's had the complete blend incorporated in their masks or not. If it was shared with the thieves. Or vice versa. Maybe it was option #C?? Where the thieves were in fact the doctors. Since there was no real cure for the plague at the time.
As with all things, it seems humans are all too often driven by their own vanity and greed. So, who knows.
One thing I know, I had a friend of mine Jeff Tree tell me about a story he has heard about the thieves blend oil which is created by blending Clove, Lemon, Cinnamon, Eucalyptus Radiata, and Rosemary together to form a strong antiseptic and highly effective tonic which prevents infections and disease from entering the body when you soak yourself in it. All of them are quite powerful and effective in and of their own cause. But when you mix them together, look out!
As the story goes. Some of the thieves who could be seen rummaging about in the plague filled wastelands that were England and large parts of Europe, were known to be somewhat impervious to the plague. So law enforcement managed to get their hands on one or a few of the thieve's. Upon pressing them to share their secrets, the thieves laughed and spit in the faces of the enforcer's. Many slanderous words were passed, names were called, mothers were torn from their deathbeds, and bamboo shims were driven under the toe nails of the thieves. But still, they would not share the secrets to their fortuitous nature.
It wasn't until years later, that the thieves last dying wish was for their great grandchildren to carry on that honoured tradition of creating their life sustaining oil for generations to come. In the eventuality that the bubonic plague would rise once more. Until the day came that they indeed found the cure for the plague. Which left the descendants of those seven thieves with that most sacred of all recipes, which we now use in our daily lives as house hold cleaning agents, personal care products, toothpaste flavour, and dish soap. The oil can also be vaporized or atomized which can then be ingested through our human orifices as a way of treating congestion and, and relieving a dry throat.
A majority of this last last paragraph was either written entirely from my own imagination, or only in part. Since we know that it still has its place in the hearts and shelves of so many people.
Naturally the story will live on through steam punk enthusiasts: 
Jim Henson enthusiasts: 
retro video gamers, and MAD magazine fans alike: 