Hantavirus is thought to generally be able to persist for a variable time in the environment- depending on a number of conditions like humidity, temperature, sunlight and even the diet the rodent had. Indoors is certainly a higher risk than an outdoor exposure. At room temperature, virus is likely viable at least 2-3 days.
Overall infectious viral survival is somewhere from hours to a few days- so that you certainly would need an active and ongoing rodent infestation to become infected- not one from old mice/rats as you seem to be describing.
The infection is contracted from breathing in contaminated air- contaminated by rodent urine, droppings and saliva, or rarely from rodent bites or other indirect contact with droppings.
The incubation between infection and symptoms is generally 2-3 weeks but can be anywhere from 1-5 weeks. If you do develop symptoms like fever, muscle aches and shortness of breath, you should seek medical attention. Frankly I would suspect your risk of hantavirus here is quite low , however.
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Edited by TheGermDoctor on 10/14/2009 at 8:19 PM EST