Complex sentence: It colonizes and infects only humans, and has never been isolated from other animals; this is thought to stem from the bacterium's inability to get iron other than from the human sources transferrin and lactoferrin.
Split into simpler sentences: It colonizes and infects only humans, and has never been isolated from other animals. This is thought to stem from the bacterium's inability to get iron other than from the human sources transferrin and lactoferrin.