A few facts:
1) fleas are very host specific, for example rat fleas cannot reproduce on a dog or cat (nor cat fleas on a dog or vice versa)
2) bubonic plague is extremely rare, especially in the US
3) rats are really very clean animals (compared to, for example, the common housefly), not that you want them around
If there are trash piles or garbage heaps in the area, that is the rat's best hideout, and they may only be visiting, in which case keeping all the food away might be enough. If they are settled in and breeding, you may need to resort to poison bait. Be very careful with that (obviously), and keep it available even after the rats are dead because others are sure to come by and setup housekeeping if these did.