Which of the Following Statements Regarding the Demonstration of the Etiology

Which of the following statements regarding the demonstration of the etiology of disease is FALSE?

A) The suspect agent must be isolated and cultured in the laboratory.
B) It must be possible to reisolate the suspect agent from the infected experimental host.
C) The suspect agent must be present in all cases of disease.
D) The suspect agent must cause the disease under investigation when introduced into a susceptible host organism.
E) The suspect agent must be the only potential pathogen present in disease cases.