is a term used by William Howard Hay, M.D. in his book A New Health Era (1933) to refer to what he considered the most fundamental cause of disease; i.e., accumulation of excess acid in the body (acidosis) which occurs when the body's intake and production of acid residue exceeds its ability to adequately eliminate it.

The theory (still promoted by Victor Earl Irons) that, as a result of intestinal stasis, intestinal contents putrefy, toxins are formed and absorbed, which causes chronic poisoning of the body. This theory was popular around the turn of the century but was abandoned by the scientific community during the 1930s. No such "toxins" have ever been found, and careful observations have shown that individuals in good health can vary greatly in bowel habits.

Toxins, which are produced by the body, when not eliminated, settle into different organs (usually the weaker organs) and result in a form of self-poisoning.