[Infectious diseases of the lower gastrointestinal tract and their differential diagnoses]

Infectious colitis comprises histologically a spectrum from normal mucosa to severe inflammation with ulceration. In many cases, a diagnosis is made microbiologically or virologically. Nevertheless, histology still plays an important role, given its potential time and cost-advantages as well as its possibility to provide a statement on differential diagnoses, leading to different therapeutic treatment strategies (anti-infective drugs, surgery, immunosuppressive agents). The pathologists' role is especially important in three scenarios: 1) The infectious agent is directly visible by light microscopy. 2) The infection shows a specific or diagnostic morphological feature. 3) The infection shows a nonspecific histological inflammation pattern and must be distinguished from important differential diagnoses, most notably chronic inflammatory bowel diseases. In this article, examples from all three diagnostic groups, their diagnostic criteria and, differential diagnosis are discussed.