Chlamydia can host other bacteria and viruses; and other bacteria, viruses, and parasites can host chlamydia, as ecto-symbionts or endo-symbionts. Ecto-symbionts are pathogens that can attach to the outside of another pathogen and endo-symbionts can live inside another pathogen. Whether a pathogen becomes an ecto-symbiont or an endo-symbiont is determined by size, and the ability of the pathogen to invade the pathogen or parasite. Larger pathogens may become an ecto-symbiont to smaller pathogens, and smaller pathogens may become an endo-symbiont to larger pathogens.

Ecto-symbionts and endo-symbionts of other pathogens and parasites can work synergistically to cause chronic disease. When a pathogen becomes an ecto-symbiont or endo-symbiont of another pathogen or parasite, the pathogen can spread to locations the pathogen may not be able to reach alone. Immortal pathogens and parasites, acting in combination, can extend the spread of the pathogens and the speed at which chronic disease develops.

The ability of an immortal pathogen to become a parasite of another pathogen complicates research and inquiry into root causes of chronic disease, by confounding any search for a single cause.