Urinary tract infections (UTIs) can be divided into upper tract infections, which involve the kidneys (pyelonephritis), and lower tract infections, which involve the bladder (cystitis), urethra (urethritis), and prostate (prostatitis). However, in practice, and particularly in children, differentiating between the sites may be difficult or impossible. Moreover, infection often spreads from one area to the other. Although urethritis and prostatitis are infections that involve the urinary tract, the term UTI usually refers to pyelonephritis and cystitis.
Most cystitis and pyelonephritis are caused by bacteria. The most common nonbacterial pathogens are fungi (usually candidal species), and, less commonly, mycobacteria, viruses, and parasites. Nonbacterial pathogens usually affect patients who are immunocompromised; have diabetes, obstruction, or structural urinary tract abnormalities; or have had recent urinary tract instrumentation.
Other than adenoviruses (implicated in hemorrhagic cystitis), viruses have no major contribution to UTI in immunocompetent patients.
The predominant parasitic causes of UTIs are filariasis, trichomoniasis, leishmaniasis, malaria, and schistosomiasis. Of the parasitic diseases, only trichomoniasis is common in the US, usually as a sexually transmitted disease (STD).