Scotsman Alexander Fleming discovers (unstable) penicillin (for typhoid, malaria, and tuberculosis). Penicillin is probably the single most important contribution to increasing life expectancy worldwide in the 20th century. The discovery is accidental and serendipitous: he interrupts his studies on staphylococci to go on vacation, forgetting an open container. Upon his return, he finds it covered in a greenish mold that had killed the pathogenic bacteria. Thus, penicillin was discovered.
