Diseases and conditions are the processes that cause the body to function in a way that is not the usual state for an individual. A disease is an internal biological abnormality that changes that quality and quantity of life. Pathology is the study of disease. Healthcare providers aim to diagnose diseases and to treat them in order to improve the quality and quantity of life. Many diagnostic errors occur that can derail the aims of improving a patient's outcome.

Conditions are externally induced changes to the body that are not necessarily pathological. An example of this is pregnancy where many of the body's systems deviate from the normal state of the mother. Other examples are changes induced by diet, exercise, environmental exposure, and medical treatments such as drugs. Only some of these lead to a pathological state, which puts them into the disease category.

The names of diseases and conditions are often made up of multiple words. When looking up a disease, some of the leading terms may be easier to remember or look up. In this index, the names are permuted, or rotated, circularly so that each important word can be the leading term in the lookup. These permuted terms are then presented alphabetically to facilitate a manual search.

Diseases and conditions are classified by general categories such as infection or cancer. The categories are indexed by customary medical classification. Within a category, the diseases and conditions are presented in a permutation index and listed alphabetically. For instance, if the user is interested in cancer diagnoses, then the category "Cancer, Tumors, and Abnormal Cell Growth" would narrow the search to diseases and conditions relevant to this category.

The Argo diagnostic software was developed to identify the underlying disease or condition as efficiently as possible. Typically, a list of diseases or conditions will be presented with the goal of making the list as short as possible but inclusive enough that a rare or unusual diagnosis will not be missed. The correct treatment depends on making the correct diagnosis.