One of the major advances of twentieth-century medicine was the discovery of penicillin. Penicillin is a member of the class of drugs known as antibiotics. These drugs either kill (bacteriocidal) or arrest the growth of (bacteriostatic) bacteria and fungi (yeast), as well as several other classes of infectious organisms. Antibiotics are ineffective against viruses. Prior to the advent of penicillin, bacterial infections such as pneumonia and sepsis (overwhelming infection of the blood) were usually fatal. Once the use of penicillin became widespread, fatality rates from pneumonia dropped precipitously.

The discovery of penicillin marked the beginning of a...