Penicillin was discovered by complete accident in 1928 by Alexander Fleming, a medical scientist. Fleming was actually studying the different strains of staphylococci (a bacteria which causes various infections), so he had germs growing in numerous petri dishes. He added a layer of agar jelly on the bottom of each dish; this jelly would then cause the bacteria to multiply, as it was the food for bacteria. Eventually, this bacteria would develop into a spot you could see without a microscope.

Like many other scientists, Fleming had difficulty keeping germs from the air out of his bacteria cultures, and noticed this in particular in one sample. As he was looking at his samples, he noticed something important in one of them: one of his cultures had become contaminated by some kind of mold from the air. This mold, now a large spot, had a radius all around it where there was no staphylococci bacteria left. The mold had cleared the zone of bacteria surrounding it and looked like it was secreting a toxin which ate up all of the germs in its path.

Fleming immediately set to studying and growing more of what eventually became penicillium notatum, and discovered that it belonged to the same family of molds as you find on stale bread or decayed fruit. He also found that it stopped the growth of most common disease germs. Sometimes, it would completely dissolve the germs so there were no longer any around that area.