How do indoor cats get urinary tract infections?

My mom's cat is a female about 14 years old and my mom thinks she might have a UTI. She is an indoor cat that NEVER goes outside and does not have a history of UTI's...how could she get one?
Update: The reason the issue arose is because Bandit peed in the bathroom of the first floor of the house. Her litter box is in the basement.
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