The exact nature, extent and time course of chronic inflammation is variable, and depends on a balance between the causative agent and the attempts of the body to remove it. Examples include Mycobacterium tuberculosis, Actinomycetes, and numerous fungi, protozoa and metazoal parasites. Such organisms are in general able to avoid phagocytosis or survive within phagocytic cells, and tend not to produce toxins causing acute tissue damage. Infectious organisms that are not innately resistant but persist in damaged regions where they are protected from host defences. The common example here is of bacteria which grow in the pus within an undrained abscess cavity, where they are protected both from host immunity and from blood-borne theraputic agents, e.g. antibiotics.
Some locations are particularly prone to chronic abscess formation, e.g. bone, pleural cavities.
Irritant non-living foreign material that cannot be removed by enzymic breakdown or phagocytosis. For many diseases characterised by a chronic inflammatory pathological process the underlying cause remains unknown. A good example here is Crohn's disease of the intestine. Attempts at reconstructing the damaged tissue occur simultaneously with the inflammatory process. These can be considered under the general title of healing and repair .