Urinary tract infection (UTI)
Causes
Urinary tract infection (UTI) - MayoClinic.com
Urinary tract infections typically occur when bacteria enter the urinary tract through the urethra and begin to multiply in the bladder. The urinary system has infection-fighting properties that inhibit the growth of bacteria and is designed to keep out such microscopic invaders. However, certain factors increase the chances that bacteria will enter the urinary tract, take hold and multiply into a full-blown infection.
Having bacteria in the urine doesn't always signify an infection. Some people, especially older adults, may have bacteria in the urine without any signs or symptoms of infection. This condition, known as asymptomatic bacteriuria, doesn't need treatment.
Cystitis may occur in women after sexual intercourse. But even girls and women who aren't sexually active are susceptible to lower urinary tract infections because the anus is so close to the female urethra. Most cases of cystitis are caused by Escherichia coli (E. coli), a species of bacteria commonly found in the gastrointestinal tract.
In urethritis, the same organisms that infect the kidney and bladder can infect the urethra. In addition, because of the female urethra's proximity to the vagina, sexually transmitted diseases (STDs), such as herpes simplex virus and chlamydia, also are possible causes of urethritis.
In men, urethritis often is the result of bacteria acquired through sexual contact. The majority of such infections are caused by gonorrhea and chlamydia.