Avian chlamydiosis is a zoonotic disease of birds caused by the intracellular bacterium Chlamydophila psittaci. This disease is called psittacosis in humans. It may be called either avian chlamydiosis or psittacosis in psittacine birds; the term avian chlamydiosis is generally used in other avian species. Infections are particularly common among psittacine birds and pigeons, but most or all species of birds are probably susceptible. Some birds carry this organism asymptomatically. Others become mildly to severely ill, either immediately or after they have been stressed. Significant economic losses may be seen in turkeys and ducks, and high mortality can occur in clinically affected psittacines.

Humans are readily infected by C. psittaci. In 1929, exposure to imported pet psittacines caused a pandemic in the U.S. and Europe. Since that time, improved screening and control of avian infections have decreased the incidence of human disease. However, C. psittaci is difficult to eliminate entirely; sporadic cases and outbreaks continue to occur. Unusual sources of outbreaks have also been reported. In Australia, one cluster of cases was linked to outdoor activities in an environment contaminated by wild birds. In people, psittacosis is readily treated with antibiotics, but it can be fatal if it is left untreated.