Osteomyelitis is usually caused by a bacterial infection. Often it is due to Staphylococcus aureus, but it can also involve other bacteria (e.g., Streptococcus, Pseudomonas Salmonella), or a fungus. S. aureus is an increasing concern because of growing spread in recent years of drug-resistant forms known as MRSA (methicillin-resistant Staphylococcus aureus).

The infection reaches the bone in one of two ways:

• Contiguous spreading osteomyelitis. Usually starts with a bacterial infection of the skin, such as a diabetic foot ulcer, before progressing into nearby bone. This is the more common form of osteomyelitis. Other sources of injury that can directly infect the bone include puncture wounds, animal bites, traffic or factory accidents and damage inadvertently caused by surgery. Surgery to remove infected bone is often necessary, along with a long regimen of intravenous and oral antibiotics.

• Hematogenous osteomyelitis. Begins with microorganisms that infect the skin, urinary tract or another area. This infection eventually enters the bloodstream and travels to bone, often located away from the original infection site. Children usually experience hematogenous osteomyelitis in a long bone of a leg or arm. Adults are more likely to suffer infection in the spine.
  
  Hematogenous osteomyelitis can often be treated and controlled with four to six weeks of intravenous antibiotics, followed by a course of oral antibiotics. In some cases, surgery to remove infected bone may be necessary.

Osteomyelitis is more common in men, children and elders. Other factors that increase the risk of developing it include:

• Trauma to skin and bones, such as deep cuts, fractures or damage caused by cancer or radiation therapy
  
  
• Diabetes or other conditions that impair blood circulation, such as atherosclerosis or high blood pressure
  
  
• Hemodialysis (a method of removing impurities or wastes from the blood when the kidneys are unable to do so) or use of other medical devices such as a urinary catheter or central line
  
  

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• Intravenous drug abuse
  
  
• Chronic infections of the skin, urinary tract or other areas
  
  
• Orthopedic surgery, including implants of prosthetics such as hip or knee replacements
  
  
• Conditions that impair the immune system, such as AIDS, removal of the spleen or use of immunosuppressive medications
  
  
• Sickle cell anemia (genetic disorder that results in abnormal blood cells)