Mice and rats may look similar but are completely different species that function in very different ways. Mice are from Europe, just like rats, and came here on ships in the 1700s just like rats. That is where the similarity ends. Mice live in a house permanently and breed inside the walls. They enter the house through weep vents, plumbing vents, wall vents, and foundation gaps as well as open doors. They live in the house permanently and never leave. Rats do not live in the house and spend most of their time outside in their burrows. While rats will use the same entryways as mice do to get into the house they are aware of their size and the dangers of humans and only go in to find food. After that, they retreat to their burrows outside. Mice can go for up to three years without drinking water which allows them to lie in a human home and eat their food without being noticed. Rats drink water in large quantities and often, assuming there is access to food on the exterior, don’t enter the house at all. These variations are why rats are often just outside and mice are always inside. Both rats and mice will infest houses, both are dangerous and can cause illnesses in various ways. For rats, they can cause sepsis and blood poisoning if they scratch you or defecate on your food and mice can cause hantavirus in similar ways. Rats are very large and frightening and are aware of this so they avoid humans at all costs. Mice are not as big and not as frightening so they will risk being seen if it means getting food. Either way, you can certainly have them both at the same time. Both can be in your house at the same time and both can be very dangerous.
Dealing with rats is very different from dealing with mice. Mice can be treated with a chemical poison called commercial-grade rodenticide. It’s an anti-coagulant poison that causes mice, as they can go years without water, to desiccate. They can die in the walls of the house and there are no issues with it. They lose all smell and moisture and can remain there permanently. A rat dying in your wall will cause massive issues, even after eating rodenticide as they drink water frequently. They will rot and spell terrible and even leave a stain on the outside of the wall that cannot be cleaned and will be dangerous to touch. Mice are treated on the interior for this reason while rats are treated on the exterior where their burrows are located. These are often referred to as areas of high activity. If you are seeing an area in your house, like the kitchen or the basement with a lot of feces then that would be an area of high activity where a licensed exterminator would place a base station for mice or a large commercial grade bated snap trap for rats. As mentioned if a rat dies in the walls it will be a terrible situation, so to avoid these snap traps are laid down to catch them before this happens. Mice can die in the walls as long as they are feeding on the rodenticide in the bait stations. So if you are getting treatment for either make sure to store your food in airtight hard plastic or glass containers to avoid giving the rodents an alternate food source.