Causes of Septic Arthritis
Septic arthritis can be caused by bacteria, viruses, and fungi. The most common causes of septic arthritis are bacterial, including Staphylococcus aureus (staph) and Haemophilus influenzae. In certain "high-risk" individuals, other bacteria may cause septic arthritis, such as E. coli and Pseudomonas spp. in intravenous drug abusers and the elderly, Neisseria gonorrhoeae in sexually active young adults, and Salmonella spp. in young children or in people with sickle cell disease. Other bacteria that can cause septic arthritis include Mycobacterium tuberculosis and the spirochete bacterium that causes Lyme disease.

Viruses that can cause septic arthritis include hepatitis A, B, and C, parvovirus B19, herpes viruses, HIV (AIDS virus), HTLV-1, adenovirus, Coxsackieviruses, mumps, and Ebola. Fungi that can cause septic arthritis include Histoplasma, Coccidioides, and Blastomyces.

Who is at risk of developing septic arthritis?
While joint infection occasionally affects people with no known predisposing risk factors, it more commonly occurs when certain risk situations are present. Risks for the development of septic arthritis include taking medications that suppress the immune system, intravenous drug abuse, past joint disease, injury, or surgery, and underlying medical conditions and diseases including diabetes, alcoholism, sickle cell disease, rheumatic diseases (for example, rheumatoid arthritis), and immune deficiency disorders. People with any of these conditions who develop symptoms of septic arthritis should promptly seek medical attention.