Catheter-associated urinary tract infection (CAUTI) is the most common healthcare-associated infection, accounting for approximately 75% of all hospital-acquired UTIs. The indwelling urinary catheter bypasses normal host defense mechanisms: the urethral sphincter, urinary flow flushing bacteria, and secretory IgA in urethral mucus. Bacteria colonize the catheter by two routes: extraluminal migration along the external catheter surface from periurethral colonization (most common in women, occurring within 24-48 hours) and intraluminal migration from contamination of the drainage system (inadequate drainage bag maintenance, disconnection of closed system). Within hours of catheter insertion, bacteria adhere to the catheter surface and form biofilm—a structured community of organisms embedded in an extracellular polysaccharide matrix that confers resistance to antibiotics and host immune responses. The biofilm provides a reservoir for ongoing bacteriuria. Escherichia coli is the most common pathogen, followed by Klebsiella, Enterococcus, Pseudomonas, and Candida (especially with antibiotic use). CAUTI risk increases approximately 3-7% per day of catheterization, making duration the single most important modifiable risk factor.