Bacteria in bladder
usually reach bladder
via the urethra, although occasionally they reach kidney via blood stream. Not emptying bladder after sex
is a common cause of utis, sub-optimal toilet hygiene-wiping back to front or keeing thighs closed with urinating can trap urine
in vagina and behind labia making nice environment for bacteria.
Bubble-bath and tub baths rather than showers are all causes.