Disease is the state or process of abnormal function in an identifiable component of the body. The affected region may include specific organs, such as the heart or lung; specific cells, such as the pancreatic beta cells or helper T cells; or specific molecules, such as in an enzyme deficiency.

Disease is most commonly associated with foreign agents, called pathogens. Bacteria and viruses are among the more popularly known pathogens. These infectious agents attack and reproduce in a host, causing disease through direct action against the body or through the body's symptomatic reaction to their presence.

Other contributors to disease include genetic predisposition and environmental influences. Inborn defects and spontaneous mutations comprise the spectrum of genetic disease. This includes cancers or autoimmune disorders. Depending on the interplay between the type of mutation and environmental cues, a genetic disease may be manifested according to different degrees of severity. There may also be a variation in the frequency of expression of the genetic disease within a population. Many metabolic disorders have genetic causes.

Nutrition plays an important role in disease. Deficiencies in vitamins and minerals have broad consequences, while excessive consumption and obesity are also associated with an array of disorders.

Classification of Diseases

There are several classification schemes for different disorders. One system focuses on the etiology or cause of the disease. This would include categories such as genetic, bacterial, and nutritional disorders. Another system focuses on the affected region. There are pathologies that affect the heart, lung, etc. Moreover, a classification system can group diseases according to type. Cancers, immune disorders, and metabolic diseases are few examples.

Due to the number and complexities of diseases, no scheme can strictly encompass the breadth of diseases. Disorders may have multiple causes and effects, as well as different affected regions and variable degrees of disease.