The answer to 'what causes rat' is the presence of food, water, and shelter. Rats are opportunistic scavengers that thrive in environments where they can easily find these basic necessities. They are drawn to homes and buildings that provide access to food scraps, pet food, standing water, and cozy nesting spots.
Rats are highly adaptable and can squeeze through surprisingly small openings to gain entry. They are skilled climbers and can access upper floors and attics. Once inside, they will build nests, breed rapidly, and contaminate food preparation areas with their droppings and urine.