In the mid-19th century, scientists isolated a bitter substance in the olive leaf and named it oleuropein. This chemical is what makes the olive tree resistant to insect and bacterial damage. Oleuropein is removed from olives when they are cured, so eating olives alone is not going to supply you with this remarkably curative substance.
In 1962, about 100 years later, an Italian researcher reported that ingesting oleuropein lowered blood pressure. Other European researchers confirmed this. They also discovered that olive leaf extract could increase blood flow in coronary arteries, relieve arrhythmias, and prevent intestinal muscle spasms, an important element in treating hepatitis C victims.
A Dutch researcher further determined that the active ingredient in oleuropein is a substance called elenolic acid. In the late 1960s, when drug experimentation was at its best, scientists at Upjohn, a major American pharmaceutical company, demonstrated that elenolic acid inhibited the growth of viruses. In fact, it stopped every virus that it was tested against.
Medical researcher Morton Walker, D.P.M., wrote about olive leaf extract in the July 1996 issue of the Townsend Letter for Doctors and Patients: "the intake of flavonoids is correlated with a lower incidence of cardiovascular disease indicating that the daily intake of olive oil and/or olive leaf extract containing phenols will likely bring on a similar result."