Complex sentence: This pathogen is specific to humans; it has not been shown to infect other animals, while other species of "Pneumocystis" that parasitize other animals have not been shown to infect humans.
Split into simpler sentences: This pathogen is specific to humans; it has not been shown to infect other animals. Other species of "Pneumocystis" that parasitize other animals have not been shown to infect humans.