Acute cystitis is irritation of the bladder. It is often caused by germs getting into the urinary tract. The urinary tract includes the kidneys, ureters, bladder, and urethra. The urethra is a tube at the bottom of the bladder. Urine flows out of this tube. The germs enter the urethra and then spread in the bladder. These germs may cause an infection in the bladder or urinary tract. This condition may also be caused by irritation from things like bubble baths, sanitary pads, sex, certain drugs, or certain foods. This condition is more common in women than men. It is also more common if you are pregnant.