In 1986, in Thailand, Thomas Grayston, M.D., thought he had found chlamydia trachoma in the eye of a child with conjunctivitis, and instead found chlamydia psittacosis (a/k/a chlamydia psittaci). Previously, psittacosis had been thought to occur only in people with bird contact. Dr. Grayston proved psittacosis can be transmitted bird-to-person, bird-to-animal, infected animal-to-people, and person-to-person—and that psittacosis can attack the eye. A study, in 1973, showed rabbits inoculated with psittacosis intravenously rapidly developed eye infections through endogenous spread.

Domestic and wild birds are the primary reservoir for psittacosis, and the pathogen has been documented in pigeons, parrots, raptors, doves, parakeets, canaries, cockatoos, cockatiels, sparrows, macaws, chickens, ducks, and turkeys. Birds can infect cats, dogs, rodents, tortoises, horses, and livestock, with psittacosis; and the animals can then spread psittacosis to people. Psittacosis has been found in humans and animals worldwide; and all strains of psittacosis are considered readily transmissible to humans.