Chronic diseases are persistently burdensome complexes of symptoms that persist for a lifetime and can progress in very different ways. This concept of disease is often defined in contrast to acute illnesses, although it is ultimately difficult to determine the exact point at which a condition becomes chronic. Age-associated changes, which correspond to a physiological aging process and certainly cause limitations, must be distinguished, as they do not strictly qualify as chronic diseases. The following section presents these various pathways to chronification.