In the early years of microbiology, scientists often removed disease material from one patient and introduced it into other individuals in an effort to understand what caused the disease. according to koch's postulates, what was the major flaw in their experimental process?

According to Koch's postulates, the major flaw in their experimental process was that they didn't isolate pure cultures of the suspected infectious agent. 
This is the reason why they couldn't determine the cause of the disease in the first patient, given that they didn't fully conduct the experiment. Had they done everything according to Koch's postulates, they would have understood this in terms of microbiology much better.