Pathology is the study of disease.

Pathogenesis is the development of disease.

Etiology is the cause of disease.

Not all disease is due to the activity of microbial pathogens.  Some diseases are hereditary, while others are degenerative, such as muscular schlerosis, endocrine, such as hyper- or hypothyroidism, mental,  such as epilepsy and senility, immunologic such as allergies and rheumatoid arthritis or neoplastic, such as cancers.

Diseases that have no known cause are said to be idiopathic.

Robert Koch was the first doctor to develop protocols for the identification of the etiological agents of disease.  His postulates (below) provided a framework for medical science to identify most of the bacterial causes of disease and has been modified to work with viral pathogens as well.

Koch's postulates do not work well with microbes that grow poorly on artificial media such as M. leprae, when disease is the result of more than one pathogen, as is the case with liver cancer since it is caused by infection of liver cells by both hepatitis B and D viruses rather than each independently and when more than one pathogen may be independently responsible for the same disease, such as the many agents of pneumonia and meningitis.