#1 on this list (posting it to give a reason for this thread to stay in B&H
):
1. Penicillin
Alexander Fleming discovered penicillin in 1928. Of course he wasn't actually looking for it at the time- he was researching the 'flu. He noticed that one of his petri dishes had become contaminated with mould. Other scientists may have recoiled in horror at this result of shoddy work practice, but not Alexander. He chose to investigate.
Whatever this intruder was, it was killing off the Staphylococcus bug - a bug causing everything from boils to toxic shock syndrome. Eventually he identified it as the fungus Penicillium notatum and it put the knife into Staph by means of a chemical that destroyed its ability to build cell walls. Being a scientist, he thought long and hard about what to call this new chemical, a chemical released from the fungus Penicillium notatum. That's right he called it penicillin. Nice one Alex. Unfortunately naturally occurring penicillin isn't very stable and thus not very useful. Fleming had found a wonder drug, but couldn't do much with it. Luckily just three years later two Oxford researchers created a stable form and today it's one of our most important tools in the fight against disease.