Instincts: Rats are instinctively wary of things new to their environment, including rat control measures such as traps and bait, and colonize in attics, burrows, under concrete and porches, in wall voids and other hard-to-reach places.
Disease: Rats can harbor and transmit a number of serious diseases. They can also introduce disease-carrying parasites such as fleas and ticks into your home.
Access: They invade your home seeking food, water and warmth.
Contamination: Each mouse can contaminate much more food than it eats.