The preclinical phase of a disease is the period of time during the natural course of a disease where symptoms are not yet apparent, but the disease is biologically present. The concept of a preclinical phase of a disease is most widely referred to in the topic of screening.

Every disease has a natural history that represents the process of the disease in the absence of an intervention. The preclinical phase is one component of this natural history. It begins at the biologic onset of disease and ends when an individual begins experiencing disease-related symptoms. Thus, this phase is when the disease is present but the individual is asymptomatic. When symptoms appear, the preclinical phase ends, and the clinical phase of the disease begins. The length ...