Final answer:
The germ theory of disease was the most important breakthrough in the development of modern medicine, pioneered by Louis Pasteur, it proposed that diseases are caused by microorganisms. Pasteur's work led to advancements in understanding and treating diseases, the development of vaccines, and improvements in hygiene.
Step-by-step explanation:
The germ theory of disease was arguably the most important breakthrough in the development of modern medicine. This theory, first pioneered by Louis Pasteur in the 19th century, proposed that microorganisms, or germs, are the cause of many diseases. Pasteur's work on proving that diseases were caused by microbes led to advancements in understanding and treating diseases, the development of vaccines, and improvements in hygiene.