The elenolic acid, present in olive leaf extract, is a derivative of the hydrolysis of bitter-flavored Oleuropein. It is present throughout the olive plant including the fruits but is concentrated above all in the leaves!
It is considered a powerful antioxidant, fighting free radicals and also facilitates blood circulation!

Elenolic acid in turn has demonstrated broad spectrum antimicrobial effects. Most of the studies quoted apply to in vitro testing. Clinical trials of olive leaf extract in the treatment of particular diseases have not been done. It is, therefore, not an approved drug or treatment for any particular disease. Nevertheless, it has been widely used as a common treatment for numerous infectious conditions supported by favorable anecdotal information.  Some viruses inhibited by olive leaf extract in vitro include rhinovirus, myxoviruses, Herpes simplex type I, Herpes simplex type II, Herpes zoster, Encephalomyocarditis, Polio 1, 2, and 3, two strains of leukemia virus, many strains of influenza and para- influenza viruses.

In in vitro testing, olive leaf extract has been reported to be an effective antimicrobial agent against over 50 pathogens, including Salmonella typhi, Vibrio parahaemolyticus, Staphylococcus aureus, Klebsiella pneumonia, and Escherichia col. Oleuropein has also been reported to directly stimulate macrophage activation in laboratory studies.