In this classification, stage 0 indicates the presence of soft-tissue swelling without bone involvement. Stage I refers to the extrinsic pressure effects on the intact bones in the vicinity of an expanding granuloma. Stage II results from irritation of the bone surface without actual intraosseous invasion. Cortical erosion and central cavitation occur in stage III. If the disease spreads longitudinally along a single ray, stage IV is established; horizontal spread along a single row represents stage V. Multidirectional spread due to uncontrolled infection is classified as stage VI.

Bone infections are usually due to haematogenous spread from distant infected organs. Spread of local sepsis or contamination of open wounds are less frequent routes of infection. The commonest cause of osteomyelitis is Staphylococcus aureus. The term rare bone infections refers to diseases where only a few percent affect bone or diseases which are essentially rare; these include bacteria, fungi, parasites and non-specific conditions. Common examples are tuberculosis, salmonellosis, brucellosis, hydatidosis, madura, actinomycosis, aspergillosis and American fungal infections. Certain bone infections have become exceedingly rare, particularly atypical mycobacteria, viral embryopathies and spirochaetes. Rare bone infections are encountered in many parts of the world commonly in the tropics and in the U. S. Immunocompromise and ease of travel can lead to increased incidence. A high index of clinical suspicion is necessary for diagnosis. Specific laboratory diagnosis is not always possible. Radiographs, computed tomography, isotope studies and magnetic resonance are useful but may not make the diagnosis. Aspiration or biopsy is necessary. Rare bone infections may simulate non-infective bone lesions.

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Twenty-five percent of the world's population could be suffering parasitic infestation. Highest prevalence is in underdeveloped agricultural and rural areas in the tropical and subtropical regions. In some areas incidence may reach 90% of the population. In contrast, some major economic projects intended to promote local development have, paradoxically, caused parasitic proliferation, e.g. bilharziasis in Egypt and Sudan and Chagas disease in Brazil. The commonest cosmopolitan gastrointestinal parasite is Entamoeba histolytica. Some intestinal parasite are endemic in temperate climates, e.g. Entrobius vermicularis. The AIDS epidemic has increased the prevalence and severity of parasitic disease, particularly Strongyloides stercolaris. Tropical parasites are seen in Western people who travel to tropical countries. Radiology has acquired a major role in diagnosis and management of gastrointestinal parasite infestations and their complications.