Definition of Onset

Onset: In medicine, the first appearance of the signs or symptoms of an illness as, for example, the onset of rheumatoid arthritis. There is always an onset to a disease but never to the return to good health. The default setting is good health.

The onset of a disease helps define it and, often, name it. Diseases are often characterized by their age of onset as congenital, infantile, juvenile, or adult. For example, in the case of diabetes, there is juvenile-onset diabetes, adult-onset diabetes, and maturity onset diabetes of the young.

Diseases may also be characterized by their speed of onset (and quickness of course) as acute (sudden), subacute (midway between sudden and gradual) or chronic (gradual). For example, in the case of leukemia, there is acute lymphocytic leukemia (ALL) and chronic lymphocytic leukemia (CLL), and acute myeloid leukemia (AML) and chronic myeloid leukemia (CML).