Sewers are great sources of food and water for rats.
The pipes form a convenient underground highway that keeps rats safe from predators as they scurry around looking for food. At the height of their breeding season in August, close to half a million rats can be found in the Portland sewer system. Occassionally rats will make a wrong turn and end up in someone's toilet. The rats are probably following a food trail created by kitchen garbage disposals or uncapped pipes. Multnomah County Vector Control gets approximately 100 calls a year about this problem.