Dr. Alexander Fleming worked in St. Mary’s Hospital in London in 1928. Having returned from Scotland, he was examining some colonies of Staphylococcus aureus. He suddenly noted that a mold called Penicillium notatum had contaminated his Petri dishes. He began studying the dishes under the microscope. Subsequently, Dr. Fleming found that some factor in the Penicillium mold not only inhibited the growth of the bacteria but, more important, it could be harnessed to combat infectious diseases.

Fleming wasn't planning to discover the first antibiotic ever and, of course, he didn't mean to revolutionize world's medicine. But that was exactly what he did that day. Anyway, after his unexpected discovery Fleming became became probably the best known pharmacologist in the world.

More Info: en.wikipedia.org