About half of all women will experience urinary tract infections in their lifetimes, and despite treatment, about a quarter will develop recurrent infections within six months of initial infection. A kind of bacteria found in the vagina may trigger recurrent UTIs, according to researchers. The findings help explain why sexual activity is associated with UTIs. When it gets into the bladder, the vaginal bacteria Gardnerella vaginalis causes dormant E. coli from a previous infection to start multiplying again, causing another UTI. Gardnerella may also contribute to more serious kidney infections.