Source : http://www.uga.edu/fruit/olive.htm
Olive - Olea europaea
“Olive oil is an important component of the Mediterranean diet, and in fact is included in the European food pyramid. Those eating the Mediterranean diet (rich in olive oil, fruits, vegetables, and fish) are known to have lower rates of colon, breast, and skin cancer, and coronary heart disease.  The active principals in olive oil are thought to be monounsaturated fats (primarily oleic acid), squalene, and phenolic compounds that function as antioxidants in the body.  Oleuropein, responsible for the bitterness of raw olives, is one of the phenolics.  Other simple phenols (e.g., tyrosol) and lignans (pinoresinol) also function as antioxidants.  Extra virgin oils are higher in these protective compounds than processed oils.  Olive oil may act by reducing the LDL (“bad”) and raising the HDL (“good”) forms of cholesterol in the blood.  Olive extracts have been shown to have hypoglycemic activity, and olive oil reduces gallstone formation by activating the secretion of bile from the pancreas.  Olive oil may act as a mild laxative.”

Source: The Mediterranean Diet Cookbook, by Nancy Harmon Jenkins, Bantam Book, 1994, pp 18-19.
“Olive oil is not the only healthy factor in the Mediterranean diet by any means, but scientists suspect that it is one of the most significant.  Why that is so takes a little explaining.
     As nutritional science evolves, so does our knowledge about olive oil, its physical characteristics and chemical composition and its impact on human metabolism.  What is clear at this point is that olive oil is unusually high in monounsaturated fats and a very good source of the elusive tocopherols (vitamin E).
     Vitamin E is a powerful antioxidant; like beta-carotene and vitamin C, it fights against radicals, damaging elements that seem to suppress the immune system and may contribute to heart disease, cancer, and lung disease as well as to the aging process.  Free radicals are produced in the body by pollutants like tobacco smoke and fuel exhaust, but they are also a normal, constant, and inevitable by-product of human metabolism.  Anything that inhibits the formation of free radicals is likely to be beneficial in disease prevention.”