In 1413, as the bubonic plague decimated France, four thieves were arrested for robbing dead and dying plague victims - a crime punishable by burning alive. The judge offered them leniency for their terrible crimes if they would share the secret which enabled them to expose themselves to the plague without contracting it.
The thieves explained that they were perfume makers and spice merchants who were unemployed due to the closure of France’s sea ports. They had prepared a special mixture of essential oils which they applied to their hands, ears, feet, masks, and temples and this protected them from infection.
As promised, the judge did not burn the men alive - he hanged them instead. Soon after, plague doctors began to wear beak-like masks stuffed with absorbent material soaked in Thieves’ Oil to protect them from disease.