The most common type of UTI occurs in the lower urinary tract, infecting the urethra and bladder. Highly virulent strains, if left untreated, can spread further up to the ureters and the kidneys, in the upper urinary tract. The symptoms of upper UTI are considerably worse, and may include back pain, nausea and fever. Fever may also indicate that bacteria have made their way into the prostate.
If left untreated, an upper UTI can cause damage to the kidneys and there is also a risk that the infection enters the bloodstream and spreads further, this is called urosepsis and calls for intensive care. So, if you suspect a UTI you should seek medical attention, in order to get a proper diagnosis and treatment. The sooner you get help, the easier your road to recovery.