Blood carries life, yet the sight of it makes people faint. It is a waste product and a commodity pricier than oil. It can save lives and transmit deadly infections. Each one of us has roughly nine pints of it, yet many don`t even know their own blood type. And for all its ubiquity, the few tablespoons of blood discharged by 800 million women are still regarded as taboo; menstruation is perhaps the single most demonized biological event.