Koch's postulates were developed in the 19th century as general guidelines to identify pathogens that could be isolated with the techniques of the day. Even in Koch's time, it was recognized that some infectious agents were clearly responsible for disease even though they did not fulfill all of the postulates. Attempts to apply Koch's postulates rigidly to the diagnosis of viral diseases in the late 19th century, at a time when viruses could not be seen or isolated in culture, may have impeded the early development of the field of virology. Koch's postulates have been recognized as largely obsolete by epidemiologists since the 1950s, so, while retaining historical importance and continuing to inform the approach to microbiologic diagnosis, they are not routinely used to demonstrate causality.