Final answer:
Louis Pasteur played a critical role in proving that diseases were caused by microorganisms, leading to improvements in hygiene and the development of vaccinations, thus substantiating the germ theory of disease.
Step-by-step explanation:
The French chemist Louis Pasteur was instrumental in the development of the germ theory of disease. His pioneering work began with experiments on the process of fermentation in 1854, which led to the discovery that microorganisms were responsible for this process as well as food spoilage and certain diseases. Pasteur's ground-breaking research disproved the idea of spontaneous generation and demonstrated through his work that microscopic beings, or germs, were the cause of diseases and could be controlled to prevent infections.
This was further substantiated by his development of the process of pasteurization to inhibit the growth of microbes in foodstuffs. Moreover, Pasteur's contribution to vaccinations, including those against anthrax and rabies, provided practical applications of his germ theory, which significantly decreased deaths from infectious diseases and improved public health and hygiene standards.