Septic arthritis is a joint infection that constitutes a medical emergency because bacteria rapidly destroy articular cartilage through enzymatic degradation within 24-48 hours if untreated. The most common pathogen is Staphylococcus aureus (including MRSA) in adults. Neisseria gonorrhoeae is most common in sexually active young adults. Infection reaches the joint via hematogenous seeding (most common), direct inoculation, or contiguous spread. The synovial membrane lacks a basement membrane, making it vulnerable to bacterial seeding. The inflammatory response (neutrophil influx, protease activation) causes rapid cartilage destruction. Diagnosis requires arthrocentesis: synovial fluid WBC >50,000 with >90% PMNs is highly suggestive; Gram stain positive in 50-75% of non-gonococcal cases; culture is the gold standard. Treatment: emergent joint drainage plus IV antibiotics.