You either get urinary tract infection from bacteria retrograding from the outside
against the bladder, or sometimes you can have bacteria circulating through the bloodstream in the body. That doesn't happen very often but the immune defenses in the bladder are not what they should be so the bacteria set up shop and becomes an issue. Untreated, it can actually go up into the kidneys which can cause irreparable kidney damage. They have high fever, they feel sick, and they don't feel like eating.
They would need therapy and antibiotics sometimes high-powered antibiotics to get over that. It's more frequent in females and usually, you see them because they're urinating very frequently. You see it in urinalysis and you treat with antibiotics and they'll be fine. With males, you don't see urinary tract infections as often because their urethra passes out of their bladder, goes around their pelvis and comes out of their penis.
Their urethra is 2-3 times longer in a male dog than a female dog of comparable size and breed so it's a lot harder for the bacteria to retrograde up there. However, with males comes the added complication because along that path is the prostate glands. Sometimes, you have to have antibiotics from 4-6 weeks because you're trying to treat all the nooks and crannies of the prostate glands to try to get out all the bacteria out of there. That's one more reason to spay or neuter your pets. Spaying or neutering your pet can resolve multitudes of complications than not performing these vital procedures.