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Variability in expression of a specific event results in a pattern appearing between extremes which is variously described in terms of spectrums, ranges, or gaussian distribution curves. Even with an event of wide variability, the expression occurring most frequently tends to be singled out as a reference standard by which the event is usually recognized or even characterized.
So it is also with the biologic event of disease. There are diseases with highly variable manifestations (eg, syphilis, allergic reactions to drugs), while other have patterns of more limited expression. It is, however, important to recognize that such a comparative statement is permissible only because the etiology of a disease is known, or its pathologic manifestations are sufficiently distinct to make it a disease entity separable from other diseases which it may casually resemble.
In the category of cancer, etiology is usually unknown, and data on pathogenesis are meager at best.
Variation in Disease Expression? Arch Dermatol. 1964;89(6):784. doi:10.1001/archderm.1964.01590300012004
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