Heresies behave like cold sores (or "the recrudescence of defeated heresies")

Heresies, like cold sores, are often succesfully treated but rarely eradicated. Instead of becoming extinct they lie dormant. Even when they have been fought off they manage to find, at some future point, conducive conditions in which to grow again. Chances are that if you meet a new heresy in the church someone has already believed and taught it.

Richard Muller has a good observation on this point in relation to the doctrine of the Trinity:

Whereas there was much debate and much very heated polemic in the sixteenth and seventeenth centuries over aspects of the doctrines of Scripture and the divine essence and attributes, the doctrines were not formulated in the context of a large-scale assault on their basic concepts, at least not until the mid-seventeenth century, after orthodox Protestant dogmatics had been fully formulated. Certainly after 1550, the opposite was the case with the doctrine of the Trinity.

The seventeenth-century orthodox formulation of the doctrine was accomplished with constant polemic against antitrinitarian views--views that grew out of a highly biblicist antrinitarianism such as Christianity had not seen since the patristic period.

Richard Muller, PRRD: volume 4 The Triunity of God, p. 19