Koch's postulates are a set of criteria established by Robert Koch in the late 19th century to identify the causative agents of infectious diseases. The postulates state that a specific microorganism must be found in all cases of the disease, isolated from the host, and then capable of causing the same disease when introduced to a healthy host. These guidelines helped lay the foundation for modern microbiology and infectious disease research. While they have been crucial in identifying many pathogens, some exceptions exist, such as asymptomatic carriers and viral infections, which may not fit all the criteria.