[The "second disease" and "second nature"].

An idea about the second disease as a metamorphosis when the phenomenon of one nature is transformed into phenomenon of the second nature is proposed. There are three stages in the development of the second disease: origin, when clinical and morphological features of initial and new nosologic forms coexist; formation-distinct groups of features of a new nosologic form which preserved a link to the first disease; separation of this new nosologic form with its typical clinical and morphological features. The second disease is metamorphosis of the first disease which develops into a new nosologic entity under the influence of exogenous or endogenous factors.