Osteomyelitis describes any inflammation of a bone or bone marrow. However, the only thing that can cause such inflammation is an infection, usually by a bacteria and such it may be treated as the equivalent of a bone infection.
Because of the structure of bone tissue, it is both difficult to infect bone and to treat infection once it establishes itself. It can be caused by either blood borne infection, a spread of infection from surrounding areas, or trauma to the bone such as a break or even the placement of replacement joints in a bone. Once bone is infected, white blood cells stream to the infection, but as part of the process of attempting to engulf the infection, the cell walls of bone or bone marrow cells can be damaged by the enzymes used in the process. This causes a build-up of pus, which restricts blood flow to the bone. As such, the infection becomes chronic as the body loses the ability to reach the area to fight off the infection, leading to necrosis. In response to the necrosis, the tissues often try to create new bone to surround the dead tissue and the area sclerosizes, becoming very hard and often leaving the bone deformed.
Osteomyelitis can happen in people of any age. In children, it most usually affects the long bones in the arms and legs, which can lead to permanent deformation of those bones. In adults, the vertebrae and pelvis are more likely to be affected. It is more common in patients who are immunosuppresed.