The "Biology-First" Hypothesis: Functional disorders may begin and end with biology-A scoping review.

While it is generally accepted that gastrointestinal infections can cause functional disturbances in the upper and lower gastrointestinal tract-known as postinfectious irritable bowel syndrome (PI-IBS) and functional dyspepsia (PI-FD)-it has still not been widely recognized that such an infection can also initiate functional non-intestinal diseases, and that non-intestinal infections can provoke both intestinal and non-intestinal functional disturbances. We conducted a scoping review of the respective literature and-on the basis of these data-hypothesize that medically unexplained functional symptoms and syndromes following an infection may have a biological (genetic, endocrine, microbiological) origin, and that psychological and social factors, which may contribute to the disease "phenotype," are secondary to this biological cause. If this holds true, then the search for psychological and social theories and factors to explain why one patient develops a chronic functional disorder while another does not is-at least for postinfectious states-misleading and detracts from exploring and identifying the true origins of these essentially biological disorders. The biopsychosocial model may, as the term implies, always begin with biology, also for functional (somatoform) disorders.