How did we accidentally discover antibiotics?

The answer

In 1928, Alexander Fleming returned from vacation to find that a mold (Penicillium notatum) had contaminated one of his bacterial cultures. He noticed a clear zone around the mold where bacteria couldn't grow. This accidental observation led to the isolation of penicillin, the first true antibiotic. It took another decade of work by Howard Florey and Ernst Boris Chain to mass-produce it. By World War II, penicillin was saving thousands of soldiers' lives and ushered in the antibiotic era of medicine.