Osteomyelitis (OM) is the infectious inflammation of bone marrow. Symptoms may include pain in a specific bone with overlying redness, fever, and weakness. The feet, spine, and hips are the most commonly involved bones in adults.

The cause is usually a bacterial infection, but rarely can be a fungal infection. It may occur by spread from the blood or from surrounding tissue. Risks for developing osteomyelitis include diabetes, intravenous drug use, prior removal of the spleen, and trauma to the area. Diagnosis is typically suspected based on symptoms and basic laboratory tests as C-reactive protein and erythrocyte sedimentation rate. This is because plain radiographs are unremarkable in the first few days following acute infection. Diagnosis is further confirmed by blood tests, medical imaging, or bone biopsy.

Treatment of bacterial osteomyelitis often involves both antimicrobials and surgery. Treatment outcomes of bacterial osteomyelitis are generally good when the condition has only been present a short time. In people with poor blood flow, amputation may be required. Treatment of the relatively rare fungal osteomyelitis as mycetoma infection entails the use of antifungal medications. In contrast to bacterial osteomyelitis, amputation or large bony resections is more common in neglected fungal osteomyelitis (mycetoma) where infections of the foot account for the majority of cases. About 2.4 per 100,000 people are affected by osteomyelitis each year. The young and old are more commonly affected. Males are more commonly affected than females. The condition was described at least as early as the 300s BC by Hippocrates. Prior to the availability of antibiotics, the risk of death was significant.

Symptoms may include pain in a specific bone with overlying redness, fever, and weakness and inability to walk especially in children with acute bacterial osteomyelitis. Onset may be sudden or gradual. Enlarged lymph nodes may be present. In fungal osteomyelitis, there is usually a history of walking bare-footed, especially in rural and farming areas. Contrary to the mode of infection in bacterial osteomyelitis, which is primarily blood-borne, fungal osteomyelitis starts as a skin infection, then invades deeper tissues until it reaches bone.

Acute osteomyelitis almost invariably occurs in children who are otherwise healthy, because of rich blood supply to the growing bones. When adults are affected, it may be because of compromised host resistance due to debilitation, intravenous drug abuse, infectious root-canaled teeth, or other disease or drugs (e.g., immunosuppressive therapy). The most commonly affected areas for children are the long bones,^{[failed verification]} and for adults, the feet, spine, and hips.

Osteomyelitis is a secondary complication in 1–3% of patients with pulmonary tuberculosis. In this case, the bacteria, in general, spread to the bone through the circulatory system, first infecting the synovium (due to its higher oxygen concentration) before spreading to the adjacent bone. In tubercular osteomyelitis, the long bones and vertebrae are the ones that tend to be affected.

Staphylococcus aureus is the organism most commonly isolated from all forms of osteomyelitis.

Osteomyelitis is often caused by Staphylococcus aureus. In infants, S. aureus, Group B streptococci and Escherichia coli are commonly isolated; in children from one to 16 years of age, S. aureus, Streptococcus pyogenes, and Haemophilus influenzae are common. In some subpopulations, including intravenous drug users and splenectomized patients, Gram-negative bacteria, including enteric bacteria, are significant pathogens.