Causes of Urinary Tract Infections

Most infections are caused by one type of bacteria, Escherichia coli (E. coli), which normally live in the colon. In most cases, bacteria first begin growing in the urethra and often move on to the bladder, causing a bladder infection or cystitis. If an infection is not treated quickly, bacteria could then travel up the ureters to infect the kidneys. This serious condition is called pyelonephritis. 

Microorganisms called chlamydia and mycoplasma can also cause UTIs in both women and men. These infections usually occur in the urethra and reproductive system (the uterus, or womb, and the ovaries and fallopian tubes). Unlike E. coli, chlamydia and mycoplasma can be sexually transmitted, and both partners should be treated for the infection. 

Some women have a long-lasting condition called interstitial cystitis, also known as painful bladder syndrome or frequency-urgency-dysuria syndrome. With this condition, the wall of the bladder becomes inflamed or irritated, which affects the amount of urine the bladder can hold. Interstitial cystitis can cause scarring, stiffening, and bleeding in the bladder. This complex condition is different from a UTI, and scientists do not know what causes it.