Before the discovery of microorganisms, diseases were often attributed to 'bad air' or divine punishment. The realization that tiny, invisible living organisms could cause illness (Germ Theory of Disease) fundamentally changed medicine. It led to the development of antiseptics, sterilization techniques, and the understanding of disease transmission. This knowledge directly informed hygiene practices like handwashing, sanitation, and food preservation, drastically reducing infectious disease rates and improving public health.