waste matter (as urine or sweat but especially feces) discharged from the body
This is the feces, urine, saliva, etc. of a being, in this case the rodent infected with hantavirus. So one one becomes infected by breathing all that nasty waste products of rodents.
Excreta is a generic term used to refer to any masses or fluids excreted as part of the digestive system of a living organism, usually that of humans. Hence it most commonly refers to urine and faeces, and is a visually non-stimulating euphemism for these words. It is used more commonly in medical parlance.