For most of human history, virtually everyone on the planet faced the risk of dying in epidemics of bacterial diseases that sometimes ravaged multiple continents. One such disease, Bubonic plague -- the "Black Death" -- killed an estimated 200 million people in the 14th century alone [source: BBC].
Then, in the late 1920s, a London physician named Dr. Alexander Fleming, who was trying to develop an antibacterial agent, noticed mold that had contaminated a Petri dish inhibited the growth of a pathogen he was studying. Fleming published a scientific article on his discovery in 1929, and one of his students, Dr. Cecil Paine, eventually became the first clinician to demonstrate the effectiveness of penicillin, a drug derived from the mold, against bacterial disease in human patients [source: Wong]. Since then, the use of penicillin and other antibiotics has led to dramatic reductions in the death rate from certain once-common diseases like syphilis, gangrene, scarlet fever, gonorrhea, and tuberculosis. In Sweden, for example, the death rate from one type of genital tract infection in infants dropped from one in 1,000 in 1911 to one in 100,000 births in 1970 [source: Hemminki].