Final answer:
The pathogen that could not be identified by the original Koch's postulates is Human papillomavirus (HPV).
Step-by-step explanation:
The pathogen that could not be identified by the original Koch's postulates is Human papillomavirus (HPV). Koch's postulates were developed to establish a link between a disease and a pathogenic microbe. However, viruses like HPV and prions do not meet all the criteria of Koch's postulates, including the ability to be grown in pure culture. Therefore, these pathogens cannot be identified using the original Koch's postulates.