Penicillin, 1928

In 1928, bacteriologist Alexander Fleming made a chance discovery from an already discarded, contaminated Petri dish. The mold that had contaminated the experiment turned out to contain a powerful antibiotic, penicillin. However, though Fleming was credited with the discovery, it was over a decade before someone else turned penicillin into the miracle drug for the 20th century.

http://history1900s.about.com/od/medicaladvancesissues/a/penicillin.htm - Patricia Basconcillo