Bladder infection, also called cystitis, is a bacterial infection of the bladder or lower urinary tract. It is caused by bacteria that attach to the inside lining tissue of the urinary system. Although the urinary system is designed to keep out such microscopic invaders, the defences sometimes fail. When this happens, bacteria may take hold and grow into a full-blown infection. Although any part of the urinary system can become infected, most infections occur in the lower urinary tract - the bladder and urethra.

The urinary system reacts to an infection similarly to how our respiratory system reacts to a cold. The tissues in the tract become inflamed, irritated, and swollen causing pain and partial obstruction to urine flow, similar to how our bodies react during a lung infection or sinus infection.

The condition is very common, and most often affects sexually active women age 20 to 50. Sexual intercourse may increase the risk of cystitis. However, the infection may also occur in those who are not sexually active.