What conditions must be met before a specific pathogen is proved to cause a disease?

The conditions that must be met before a specific pathogen is proved to cause a disease include; Pathogen must be present in every case in which the disease is found, pathogen must be isolated and grown outside the body in a pure culture, healthy animals infected with culture must develop the disease and finally the pathogen must be re-isolated and cultured from the newly sick animals and must be identical to the original pathogen. 

Hmmm i'm not sure about this one