Although the urinary tract system is designed to keep bacteria out, problems can occur. Escherichia coli (E. coli) or other bacteria found in the intestines can enter the urinary tract through the urethra.
These bacteria can travel up into the bladder. When this happens it can cause cystitis (inflammation of the bladder). It can also cause a urinary tract infection (UTI).

If the infection continues up to the kidneys, it can cause kidney infection. This problem can be severe.

You are more likely to get a kidney infection if you have frequent bladder infections or have a structural problem in the urinary tract. Urine normally flows only in one direction-from the kidneys to the bladder. If the flow of urine is blocked or flows in the wrong direction, infections can happen. Urine flow can be blocked by many things, including:

• Kidney stones, 
• Tumors inside or outside the urinary tract and
• Structural problems of the urinary tract.
• People with diabetes, hypertension or a weak immune system are at high risk for kidney infections.

Untreated infection can damage the kidneys permanently and lead to long term problems. In some cases, kidney infections can lead to kidney disease, high blood pressure, or kidney failure. If kidney infection spreads to the bloodstream it can cause a serious problem called sepsis- A life threatening infection.