A contagious disease of birds transmissible (especially from parrots) to human beings as a form of pneumonia. A zoonotic infectious disease caused by the bacterium Chlamydophila psittaci (formerly Chlamydia psittaci) and contracted from parrots and many other species of bird.
Psittacosis is also known as parrot disease, parrot fever and ornithosis.
The word is derived from the Psittacus genus of parrots of which the African Grey is a prime example, although the genus also includes the New World macaws.
The OED gives the first use of psittacosis as being as late as 1897, but maybe that isn't surprising as that's when much of medicine was being codified. By comparison psittac for a parrot is recorded way back around 1400.