The discovery of penicillin changed the world of medicine enormously. With its development, infections that were previously severe and often fatal, like bacterial endocarditis, bacterial meningitis and pneumococcal pneumonia, could be easily treated. Even dating all the way back to World War II and today with the war in Iraq, soldiers experienced injuries that would have been fatal without penicillin and other antibiotics that were developed subsequently. It is really impossible for me to imagine what the world would be like without penicillin. I question whether there would be a discipline of infectious diseases as we know it today.
There were beginning treatments for pneumococcal pneumonia in the 1930s with antisera and sulfonamides, but use of these treatments quickly came to a halt, and everyone began using penicillin. This quickly led to a number of pharmaceutical industries beginning to screen a variety of other natural products for antibacterial activity, which led to a whole host of new antibiotics, such as streptomycin, aminoglycosides, tetracycline and the like. Penicillin clearly led the way in that development.
It is interesting that using penicillin for the treatment of infections like pneumococcal pneumonia and bacterial endocarditis never had a randomized, controlled trial because the difference with treatment was so clearly apparent that no one even thought of doing a randomized controlled trial.