so its the go too the four ( 4 ) Koch's Postulates 

Four criteria that were established by Robert Koch to identify the causative agent of a particular disease, these include:

1. the microorganism or other pathogen must be present in all cases of the disease
2. the pathogen can be isolated from the diseased host and grown in pure culture
3. the pathogen from the pure culture must cause the disease when inoculated into a healthy, susceptible laboratory animal
4. the pathogen must be reisolated from the new host and shown to be the same as the originally inoculated pathogen

However, Koch's postulates have their limitations and so may not always be the last word. They may not hold if:

The particular bacteria (such as the one that causes leprosy) cannot be "grown in pure culture" in the laboratory.
There is no animal model of infection with that particular bacteria.

A harmless bacteria may cause disease if:

• It has acquired extra virulence factors making it pathogenic.
• It gains access to deep tissues via trauma, surgery, an IV line, etc.
• It infects an immunocompromised patient.
• Not all people infected by a bacteria may develop disease-subclinical infection is usually more common than clinically obvious infection.

Despite such limitations, Koch's postulates are still a useful benchmark in judging whether there is a cause-and-effect relationship between a bacteria (or any other type of microorganism) and a clinical disease.

What does the first of Koch's postulates state quizlet?

What is Koch's first postulate? The microorganism must be present in all cases of disease. ... The pathogen grown in pure culture must cause disease when placed in a healthy animal.