Antibiotics

Until Alexander Fleming discovered penicillin in 1928, almost any little bug that someone picked up was potentially fatal. Once penicillin—and later a whole range of other antibiotics—came on the scene, however, death due to bacterial infection became rare, resulting in a greatly reduced mortality rate and much longer life-span. It also rendered many scourges of the past—from small pox and typhoid to gonorrhea and syphilis—obsolete or, at least in the case of venereal disease, something easily treatable.

Source: http://www.toptenz.net/top-10-inventions-of-the-20th-century.php