A bladder infection is most often caused by a bacterial infection within the bladder. In people with weakened immune systems, yeast can cause bladder infections as well. A bladder infection is a type of urinary tract infection (UTI). This refers to an infection anywhere in the urinary tract, such as the bladder, kidneys, ureters, or the urethra. Most cases of bladder infections are acute, meaning they occur suddenly. Other cases may be chronic, meaning they recur over the long term. Early treatment is key to preventing the spread of the infection.