Most often in patients < 3yo
MSSA and MRSA are most common causes in all age groups
Patients with sickle cell disease, rheumatoid arthritis, and inflammatory bowel disease are at increased risk
Septic arthritis 
Trauma
Monoarticular arthritis
Septic arthritis
Reactive arthritis
Acute Rheumatic Fever
Septic bursitis
Reactive Arthritis 