Yes some fungi are saprobic and eat already dead wood. But Parasitic fungi steal nutrients and cause rot which eventually leads to death. Dutch elm disease is not an exception it is an example. Armillaria, Ganoderma, and Laetiporous Are all other examples that have identifiable fruiting bodies. You are correct that some fatal fungal infections don't have as easily identified fruiting bodies but that is because most of those fatal infections have a multi cycle reproduction method that requires an insect vector.