This is a guide to "blue-green algae", a group of five foods: spirella, chlorella, wild blue-green grass, wheat grass and barley grass. They were used for their nutrient properties by ancient cultures in Egypt and central America, and are now being harvested in freshwater lakes in Mexico and North America. Available in simple, spray-dried tablet form, they are the basis of a $1-billion-dollar worldwide health industry. Blue-green algae are used in cancer therapy and malnutrition treatment, and have destroyed the AIDS virus in test tubes. They are suitable for vegetarian diets and as a natural power-supplement for sportspeople.