Urinary tract infections in women are divided into: upper urinary tract infections and lower urinary tract infections. Any pathogenic bacteria that invade the urinary tract can cause urinary tract infections. The most common pathogenic bacterium is E. coli. The main ways for bacteria to enter the urinary tract: 1. Bacteria enter the urethra from the perineum of the host and rise to the bladder, ureter and kidney, which we call an upstream infection; 2. Bacteria enter the kidney through the bloodstream; 3. Bacteria enter the kidney through the lymphatic route. The vast majority of urinary tract infections are ascending infections. The pathogenic bacteria (mostly intestinal bacteria) first settle and multiply in the perineum and contaminate the external urethral orifice through the urethra up to the bladder. Once the bacteria enter the bladder, about 30% of them can invade the ureter to reach the renal pelvis. The body has a complete set of antibacterial defense mechanisms: 1, the defense of the bladder: the concentration of urea nitrogen and ammonium chloride in urine and its osmotic pressure is related to the bactericidal power; urea and the low pH value of urine (less than 6) constitute the main antibacterial factors of urine. 2, the flushing effect of urine: urination can remove most of the bacteria in the bladder, but only if there is no residual urine in the bladder after urination and the bacteria have not adhered to the bladder mucosa. Any bacterial infection begins with the adhesion of the causative bacteria to the host tissue, which later appears to migrate and then leads to infection. 3, Anti-reflux mechanism of the ureter. 4, The defense mechanism of the renal cortex. The female urethra is about 5-7Cm long, and the external urethral sphincter and urethral mucosa play a role in the closure of the urethra, and the morphology of different external urethral orifices has an impact on the incidence of urinary tract infections.