Penicillin is arguably the most helpful medical discovery of the 20th century. It was discovered in 1928 as the active agent in a mould that can destroy and inhibit bacteria. Humans wind up with so many nasty bacterial infections like pneumonia that previously might just kill you, now were treatable! Not only did we discover (and eventually, mass produce) this particular drug, but it also kick started a whole new family of antibiotics for treating diseases.

Of course, we do eventually wind up some issues with antibiotic resistances of bacteria and some people wind up allergic, but at the time of discovery it must have truly seemed a panacea. Note that the Penicillium genus was known to be doing something against infections since the late 1880's, but the exact mechanism had not been identified.