Risk of septic arthritis increases with anti-tumor necrosis factor alpha treatment. Immunosuppressive medication
Intravenous drug abuse
Recent joint surgery
Hip or knee prosthesis and skin infection
HIV infection
Other causes of sepsis

Organisms
Most cases of septic arthritis involve only one organism; however, polymicrobial infections can occur, especially after large open injuries to the joint. Septic arthritis is usually caused by bacteria, but may be caused by viral, mycobacterial, and fungal pathogens as well. It can be broadly classified into three groups: non-gonococcal arthritis, gonococcal arthritis, and others. Non-gonococcal arthritis – These bacteria account for over 80% of septic arthritis cases and are usually staphylococci or streptococci. Such infections most commonly come from drug abuse, cellulitis, abscesses, endocarditis, and chronic osteomyelitis. Methicillin-resistant Staphylococcus aureus (MRSA) may affect 5 to 25% of the cases while gram negative bacilli affects 14 to 19% of the septic arthritis cases. Gram negative infections are usually acquired through urinary tract infections, drug abuse, and skin infections. Older people who are immunocompromised are also prone to get gram negative infections. Common gram negative organisms are: Pseudomonas aeruginosa and Escherichia coli. Both gram positive and gram negative infections are commonly spread through the blood from an infective source; but can be introduced directly into the joint or from surrounding tissue. It often affects older people, and often happens suddenly, involving only one joint. Joint aspiration cultures are positive in 90% of cases, while only 50% of blood cultures yield any organisms.
Acute septic arthritis, infectious arthritis, suppurative arthritis, osteomyelitis, or joint infection is the invasion of a joint by an infectious agent resulting in joint inflammation. Generally speaking, symptoms typically include redness, heat and pain in a single joint associated with a decreased ability to move the joint. Onset is usually rapid. Other symptoms may include fever, weakness and headache. Occasionally, more than one joint may be involved, especially in neonates and younger children. In neonates, infants during the first year of life, and toddlers, the signs and symptoms of septic arthritis can be deceptive and mimic other infectious and non-infectious disorders.In children, septic arthritis is usually caused by non-specific bacterial infection and commonly hematogenous, i.e., spread through the bloodstream. Septic arthritis and/or acute hematogenous osteomyelitis usually occurs in children with no co-occurring health problems. Other routes of infection include direct trauma and spread from a nearby abscess. Other less common cause include specific bacteria as mycobacterium tuberculosis, viruses, fungi and parasites. In children, however, there are certain groups that are specifically vulnerable to such infections namely preterm infants, neonates in general, children and adolescence with hematologic disorders, renal osteodystrophy and immune-compromised status. In adults vulnerable groups include an artificial joint, prior arthritis, diabetes and poor immune function. Diagnosis is generally based on accurate correlation between history-taking and clinical examination findings and basic laboratory and imaging findings like joint ultrasound.In children septic arthritis can have serious consequences if not treated appropriately and timely. Initial treatment typically includes antibiotics such as vancomycin, ceftriaxone or ceftazidime.