A common way this happens is when a person breathes in the hantavirus from the air. Let's create an imaginary scenario and go through the process step by step. Say you have a storage room in your home that you hardly ever enter. You keep old furniture there, old newspapers and magazines, and so on. At some point, a group of deer mice find their way into the room, looking for places to build nests.
Some mice chew through the fabric of an old armchair and build a nest inside it.
Other mice shred bits of magazines and build nests under the shredded pieces. A few of these mice are infected with the hantavirus. The infected mice don't show any signs of being sick. Out in the environment like this, the virus can live for several days. Meanwhile, you decide to clean up your storage room.