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 reductions in the mortality rate from certain infections like syphilis, septicemia and, of course, bubonic plague [source: Hemminki and Paakkulainen]. Interestingly though, antibiotics can't claim all the credit when it comes to decreased mortality rates in common bacterial diseases. Other breakthroughs on our list, like clean water, have a big role to play, too [source: Hemminki and Paakkulainen].