Rats, Mice & Squirrels - Oh My!
Those aren't little black grains of rice on the kitchen counter! That "rice" is evidence of mice that have come indoors during winter.  Other animals that may come into our houses, barns or sheds include rats, squirrels and chipmunks. Or we may find also find evidence of raccoons or skunks. 

 
Rodents
Rodents may come in through almost any opening - pet doors, holes in walls, missing vent screens, openings around pipes, dryer ducts vents, etc. Mice can squeeze through spaces as small as a nickel.  Rats can squeeze through a space as small as a half-dollar.
Rodents can spread diseases.  House mice constantly give off hundreds of micro-droplets of urine as they travel around their territory every day. A large medical research study showed a protein in house mouse urine called mouse urinary protein (MUP), caused allergies in 18% of the inner city children studied.
They also can cause structural damage to houses by gnawing through wood, insulation or plastic.
Mice and rats are usually active at night and may be responsible for night sounds in the home.  Control measures include, exclusion - covering any openings with hardware cloth or metal sheathing, repellents, toxicants, traps, and house cats.  Note that for cats to be effective, they must have learned how to catch rodents as kittens. Ultrasonic devices have not been proven to control mice.
 
Chipmunks & Squirrels
Chipmunks assemble a collection of winter food and store it in their burrows, which (depending on the species) may be in stone walls, fences, under bushes or in turf.  Squirrels often come into homes in the fall and build nests in attics, behind soffit panels, or in wall voids.
They can get into open areas such as garages to seek food such as grass seed, bird food and pet food. They will readily eat through plastic bins to get to the food. Squirrels are active in the daytime and cause damage to homes by gnawing holes, and chewing wires.  They will also contaminate and stain surfaces with their urine and droppings. Squirrels may bring fleas, mites and lice into homes. The pine cones that squirrels store can attract and breed a wide range of secondary pests, such as beetles, moths or mites.
The best long-term control of squirrels is removal, followed by exclusion, usually by covering all entrances into the home with heavy sheet metal or hardware cloth. 
            
Source: Prevention and Control of Wildlife Damage, Cooperative Extension Division, Institute of Agriculture and Natural Resources, University of Nebraska, Lincoln, factsheets, 1994.