Chlamydia psittaci was identified simultaneously by three investigators in 1930. It is one of four species within the genus Chlamydia. Based on RNA sequencing, it is currently considered distinct from Chlamydia pneumoniae and C. trachomatis, despite phenotypic and physiologic similarities that have taxonomically bound them for many years. The organism is an obligate intracellular pathogen that contains both RNA and DNA but lacks a classic cell wall. These characteristics contribute to both the clinical manifestations and the determination of therapeutic options. C. psittaci has a wide range of host species, including birds, humans, and lower mammals. C. pneumoniae, however, is found only in humans and C. trachomatis only in humans and mice.

The systemic illness associated with Chlamydia psittaci has been termed psittacosis because of its association with parrots and psittacine birds. Subsequently, many avian species have been found to harbor C. psittaci and to transmit the organism to humans, causing disease. The term ornithosis would be more appropriate; however, it is not traditional. The organism can be carried for years in birds, remaining latent and causing disease many years after acquisition. Transmission to humans can occur even in the absence of disease in the bird. Excretion in the feces with aerosolization is the typical mode of transmission. Human-to-human transmission has been documented rarely and usually in the setting of severe disease. Healthcare workers have acquired the disease, but it is not felt warranted to isolate patients when hospitalized. Cases of mammal-to-human acquisition have been described in the setting of placental aeration at birth, but these cases are likely caused by the now separate species Chlamydophila abortus.