With some putting the "lives saved" count in the billions, Karl Landsteiner may well have saved yours. Before we had any idea about blood types, people would regularly bleed to death from all kinds of complaints - ulcers, accidents, childbirth, you name it. Basically, if something happened that caused you to bleed faster than your body could naturally replace it, then it was bad news and bad luck. Giving somebody the wrong type of blood isn't exactly a laugh a minute with symptoms starting with "an impending sense of doom" and ending with uncontrollable clotting, kidney damage and death. Doctors had attempted blood transfusions before the knowledge of blood groups, but it was sheer dumb luck if it ever actually worked. In 1901, Landsteiner discovered that blood is not all the same and that different people's blood had different characteristics that made it incompatible, nay, deadly, to others'. This transformed the risky last-resort of a blood transfusion into a routine procedure that has not only saved lives, but made previously risky medical practices a breeze.
