- Bacterial: Direct introduction via surgery, trauma, injection; hematogenous spread; Staph, Strep most common; consider Gonorrhea if STI; Reactive arthritis (post gu/gi infxn); Acute rheumatic fever
- Viral: Parvo, Chikungunya, Hepatitis B
- Spirochete: Lyme
- Worse w/movement
- One or few joints