Animals have been domesticated for more than 10,000 years. Close contact between humans and animals allowed animal pathogens to enter the human microbiome. Chlamydia pathogens are immortal intracellular animal pathogens that live inside human cells. Chlamydia pathogens were transmitted from animals-to-humans; and evolved into forms capable of transmission from animal-to-human, from human-to-animal, and from animal-to-animal. Animals can also transmit different types of parasites to humans, and shed parasites on the ground and in water sources. Cats are the only known animal-to-human vector for the toxoplasmosis parasite.