It is a infection in the Urinary Tract. The Urinary Tract Infection is also reffered as UTIs.It is usually involving  Kidneys, Urethera or Ureters because these are the parts through which urine is eleminated from the body.
Any part of this system can become infected.The farther up in the urinary tract the infection is located, the more serious it is.
Ans its main cause in 90% cases is bacteria.
So, its basically its an infection in the urinary tract and this sometimes leads to kidney stones too.

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