In medical practice, Dr. Merchant followed the principle of diagnose what you can and treat what you can diagnose, for over forty years. Repeated diagnosis of chronic infection in a variety of chronic diseases and successful treatment outcomes revealed that chronic infections cause chronic disease.  Diagnoses of chronic infections and successful treatment outcomes provided important insights into the causes of chronic diseases, the consequences of having a chronic infection, and the relationships and commonalities between chronic diseases, which is rarely available in a research setting.