Final answer:
Koch's four postulates are a series of proofs that confirmed the germ theory of disease by establishing criteria to link a pathogen to a specific disease, which has been crucial in the understanding and identification of disease-causing microorganisms, despite some noted limitations.
Step-by-step explanation:
Koch's four postulates are a series of proofs that verified the germ theory of disease. These postulates required that a microorganism had to be presented in abundance in all individuals with the disease, and not found in healthy individuals; it had to be isolated from a diseased host and grown in pure culture; then, it should cause disease when introduced into a healthy host; and lastly, the microorganism had to be re-isolated from the experimentally infected host and identified as identical to the original specific causative agent.
In essence, each of Koch's postulates was a criterion that must be met before a disease could be positively linked with a pathogen. The tests involved comparing cultures from healthy and diseased animals, and these criteria helped discover the pathogens responsible for diseases like anthrax, tuberculosis, and cholera. While groundbreaking at the time, the limitations of these postulates have since been recognized. For example, not all pathogens can be grown in pure cultures, and some can be found in healthy individuals without causing disease.