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Usually bats will leave a small pile of droppings, or guano in the center of your attic. If you don't have very many bats, or the problem is fairly new, it may look like this.
If bat droppings are really accumulating and piling up in your attic, odds are good that fungal spores levels will be quite high. These can result in serious respiratory infections. You should avoid spending time in such an attic without lung protection. Hystoplasmosis can result from breathing in too much of these fungal spores and medical attention is needed.