In the late 1800s, a researcher by the name of Robert Koch developed a series of postulates (guidelines) that can be used to prove that a given pathogen is the true cause of a disease. Koch's Postulates 1. The suspected causative agent must be found in every case of the disease and be absent from healthy hosts. 2. The agent must be isolated and grown outside the host. 3. When the agent is introduced to a healthy, susceptible host, the host must get the disease. 4. The same agent must be found in the diseased experimental host. Which of the following steps does NOT follow one of Koch's postulates?