Acute infection with intracellular bacteria is most often acquired through the eyes, nose, mouth, and reproductive tract; and causes severe acute illness lasting weeks to months. (Some immortal pathogens can cross the placenta.) Intracellular bacteria thereafter become a chronic infection that spreads in the body, over the course of years and decades, to cause chronic disease.

An immortal pathogen can have a predilection to attack particular types of tissue or organs; engage in opportunistic attacks; or hide in organs, tissue, or other pathogens, to aid survival. Chronic infection triggers an immune battle and cascade of events and reactions, to the pathogen and its by-products; and can cause a wide variety of symptoms and findings, which can change over time. Symptoms and findings can change in response to spread of the infection, co-infections, the cumulative infectious burden, environmental triggers, and prior treatment of symptoms.

Medicine has divided the symptoms and findings of chronic intracellular infection into thousands of named chronic diseases. The diagnosis of chronic disease is too often based on the specialty bias of the examiner; and the diagnosis is a description of symptoms and findings, rather than a diagnosis of the immortal pathogens causing the symptoms and findings.