Physiologist Claude Bernard develops the concept of the milieu intérieur (“internal environment”), arguing that the stability of the body's internal condition, not external agents alone, determines health. This becomes the conceptual root terrain theory builds on.

Béchamp's own early fermentation research runs parallel to — and by his own account predates — Pasteur's, becoming the seed of their priority dispute (see Origins).

Béchamp develops the microzymian theory: that the body contains “microzymas” — tiny particles he considered the fundamental units of life — capable of transforming into bacteria when the internal terrain is disturbed.

Béchamp publishes Le Sang et son troisième élément anatomique (English translation, The Blood and Its Third Anatomical Element, 1911–12), laying out the microzymian theory in full.

Benedict Lust founds the American School of Naturopathy in New York — the first formal institution built around terrain-adjacent, non-pharmaceutical principles.