Final answer:
The germ theory of disease, which proposed that microorganisms are the cause of many diseases, was first clearly stated by Girolamo Fracastoro in the 1500s and gained widespread acceptance in the late 1800s due to contributions from scientists like Louis Pasteur and Robert Koch.
Step-by-step explanation:
The theory of disease first proposed in the 1800s that stated microorganisms could cause infection and illness is the germ theory of disease. This theory was a revolutionary shift from previous beliefs, such as the miasma theory, and played a crucial role in the development of modern medicine. Girolamo Fracastoro was an early proponent of the theory in the mid-1500s, but it was not until the late 19th century that it gained widespread acceptance thanks to the work of pioneers like Louis Pasteur and Robert Koch. Koch's postulates were particularly significant in establishing a systematic approach to link specific microorganisms to specific diseases. The acceptance of the germ theory led to numerous advances in public health, sanitation, and medical practices, such as vaccination and sterilization.