About 1 in 5 women experience a second urinary tract infection (UTI), while some are plagued incessantly. In most cases, the culprit is a different strain of bacteria, but some types can invade the body's cells and form a community safe both from antibiotics and the immune system. A group of these renegades can travel out of the cells, and then re-invade, ultimately establishing a colony of antibiotic-resistant bacteria primed to attack again and again.
Some women are genetically predisposed to UTIs, while others have abnormalities in the structure of their urinary tract that make them more susceptible to infection. Women with diabetes may be at higher risk, as well, because their compromised immune systems make them less able to fight off infections like UTIs. Other conditions that increase risk include pregnancy, multiple sclerosis, and anything that affects urine flow, such as kidney stones, stroke, and spinal cord injury.