As far back as the 5th century BC the Greek physician Hippocrates wrote about the use of a bitter powder extracted from willow bark that reduced fevers and eased aches and pains. Native Americans also used an infusion of willow bark for similar purposes. What was this remarkable "healing" principle within the bark that relieved disease?
Known as salicylic acid (from the Latin salix, willow tree), this pain-killing compound is widely distributed throughout plants, where it functions as a hormone. The more vegetables and fruits you consume, the more likely you are to have a physiologically significant concentration of salicylic acid in your blood. This is why, for instance, vegans and vegetarians generally have higher levels than most grain- and meat-based consumers. 
The chemical acetyl-salicylic acid, commonly known as aspirin, is a synthetic form of salicylic acid, a compound which is formed when salicin, a bitter compound naturally found within plants like white willow bark, is broken down within the human body. Salicylic acid can also be synthesized endogenously from benzoic acid, and its urinary metabolite, salicyluric acid, has been found to overlap levels in patients on low-dose aspirin regimens. Cell research indicates that salicylic acid compounds (known as salicyclates) actually compare surprisingly well to aspirin in reducing inflammatory activity.