Complex sentence: An autopsy by Scully shows sand in his lungs, indicating the existence of a silicon-based spore; she theorizes that the spore infects the nearest hosts, or otherwise dies.
Split into simpler sentences: An autopsy by Scully finds sand in his lungs, indicating the existence of a silicon-based spore. Based on her research, Scully believes that the spore infects the nearest host, or otherwise dies.