The bubonic plague wreaked havoc in Europe off and on for about 600 years before peaking in the 1300s. Century after century, as late as the 1700s, outbreaks claimed up to half the population. The plague had a big influence on the life of William Shakespeare, having claimed the lives of some of his siblings as well as causing his theater to be shut down during several especially nasty outbreaks in London between 1593 and 1608.
It is well know that the bubonic plague is a bite-based infection. A lesser known fact is that there were many more victims than those bitten by fleas. It turns out that the bubonic plague was often the first step of a progressive series of illnesses. Two other types were pneumonic and septicemic. The resulting pneumonic plague was also very infectious and allowed person-to-person transmittion.
This is the period of time responsible for the bizarre images of physicians wearing dark robes, wide-brimmed hats, and masks with long beaks. There was actually method to the madness. These beaks held dried herbs, spices and essential oils which the physician breathed. The robe was doused with a similar fragrant concoction. Scientific evidence today is building support for this seemingly outrageous behavior.
Meanwhile in France another interesting aromatic legend developed around a horrid sounding witch's brew known as "Marseilles Vinegar" or "Four Thieves Vinegar." A variety of recipes floated around. One recalled by Scientific American in 1910 included things like dried rosemary tops, dried sage flowers, fresh rue, camphor, "spirit," garlic cloves and vinegar which was to steep for 7 or 8 days "with occasional agitation." It was said that this "medicated vinegar was invented by four thieves of Marseilles who successfully employed it as a prophylactic during a visitation of pestilence." For those who don't travel the renaissance fair circuit, "pestilence" was the medieval term for bubonic plague.
The earliest online English reference found so far is in the 1825 Pharmacologia. After recounting the story of the aromatic vinegar used by the four thieves of Marseilles, it goes on to note that, "It was, however, long used before the plague of Marseilles, for it was the constant custom of Cardinal Wolsey to carry in his hand an orange, deprived of its contents, and filled with a sponge which had been soaked in vinegar impregnated with various spices, in order to preserve himself from infection, when passing through the crowds which his splendour or office attracted. The first plaque raged in 1649, whereas Wolsey died in 1531." The Pharmacologia then sites the French Codex and The German Dispensatories as possible earlier sources of the vinegar recipes.
Was the concoction actually effective? Despite being branded a "very useless preparation" in a 1854 medical book, stories persist that indicate there were certainly positive results, if only because of the garlic. Apparently doctors who carried garlic in their pockets were protected from the plague as were French priests who ate garlic and safely ministered to the dying while the garlic-free English priests fell ill.
But the biggest twist in this tale is yet to be told. In 1966, a book called "Nature's Medicines" was published with this tasty tidbit...
"In Marseilles, a garlic-vinegar preparation known as the Four Thieves was credited with protecting many of the people when a plague struck that city (1722). Some say that the preparation originated with four thieves who confessed that they used it with complete protection against the plague while they robbed the bodies of the dead. Others claim that a man named Richard Forthave developed and sold the preparation, and that the 'medicine' was originally referred to as Forthave's. However, with the passing of time, his surname became corrupted to Four Thieves." 
Could it really be true that the infamous four thieves never existed?! Were they created out of thin air via a centuries-long game of telephone in which the original formula was ultimately as mangled as the creator's name? We may never know.
Fast forward to the end of the last century and the barely-remembered story takes its first steps toward legendhood. Dr. John R. Christopher, probably the most popular of pioneering US herbalists, attached the story to a garlic-heavy formula he created and about which he began educating people. His formula, the story of the four thieves and the possible contribution of the mysterious Mr. Forthave were all mentioned in the April 1977 "garlic" issue of his newsletter, "Concern."
It was famous French aromatherapy doctor Jean Valnet (1920-1995) who gave the story its essential oil gravitas. In his book, "The Practice of Aromatherapy," Valnet quotes the archives of the Parliament of Toulouse. He claims the original recipe was revealed by corpse robbers who were caught red-handed in the area around Toulouse in 1628-1631. Given the virulence and deadliness of the plague, the judges were astonished by the indifference of the thieves to contagion.
But D. Gary Young, founder of Young Living, is probably most responsible for the story reaching the legendary status it enjoys today. While at his clinic in Mexico, Young was introduced to the therapeutic possibilities of premium essential oils. He learned that they are hundreds of times more concentrated than the herbs and tinctures he had worked with while studying under pioneers like Norman Walker, Bernard Jenson and, yes, Dr. Christopher. The revelation reshaped his life as he saw dramatic results in his own clinical studies. He went on to start Young Living, a company which now has farms all over the world for the production of essential oils. In 1992, Young created an oil blend that was intended to support thier immune systems. As a student of both Dr. Chistopher and Dr. Valnet, it was only natural for Young to recall the four thieves story. Thieves Oil was born. One of the first published explanations of Thieves Oil was in Young's 1996 book, "Aromatherapy: The Essential Beginning."
Mounting evidence is demonstrating that the blend's effectiveness stands up to the legend and ongoing studies are also showing unparalleled protection against mold.
The legend, the studies, the pleasant aroma and taste (a welcomed difference from garlic and vinegar recipes), and the convenience have combined to make Thieves Oil the most popular blend of oil sold by Young Living and possibly the most recognized brand in the world of aromatherapy. An entire line of household and hygiene products have sprung up around it. Those who find it difficult to wrap their minds around the usefulness of either a vinegar recipe or an oil blend may better relate to Thieves household cleaner, soap or hand sanitizer. And the taste of Thieves toothpaste, mouthwash and lozenges will certainly be more pleasant than the brew offered by either the four thieves or Mr. Forthave.
Here's a History Channel documentary on the plague. It avoids any mention of Four Thieves Vinegar, but it offers decent insight on the spread of the plague and the politics surrounding it.
1. Shakespeare and the Black Plague
2. Secondary plagues spread through air, not just flea bites
3. Beak Doctor Costume
4. The Scientific American Cyclopedia of Formulas, 1910, p878
5. The New Sydenham Society's Lexicon of Medicine and the Allied Sciences, 1881
6. Medical Lexicon: A Dictionary of Medical Science, 1874, p10
7. Popular Science Monthly, Volume 30, January 1887, p383
8. The Scientific American Cyclopedia of Formulas, 1901, p585
9. Paris Pharmacologia, Volume 2, 1825, p18
10. The Elements of Materia Medica and Therapeutics, Volume 2, 1854, p946
11. Garlic Revisited: Therapeutic for the Major Diseases of Our Times? TH Abdullah, 1988, p1
12. Nature's Medicines, Richard Lucas, 1966, p38
13. Early citation of "Forthave's Vinegar"...The Mirror of Literature, Amusement, and Instruction, Volume 12, 1828, p89
14. The Dr Christopher Story as told by his son, David Christopher
15. Concern, Dr. John R, Christopher, April 1977. p7
16. Dr Jean Valnet and French Connection
17. Gary Young Blog post: "Finding Pure Essential Oils," Part 1 and Part 2
18. Gary Young Blog post: "Finding Pure Essential Oils," Part 8
19. Mark Schreuder recounts the creation of the Thieves oil blend on the Young Living Wellness Essentials Audio Series CD #4.
20. Aromatherapy: The Essential Beginning, D. Gary Young, N.D., 1996, p130
21. Thieves Oil vs Mold