Psittacosis in humans and avian chlamydiosis in birds are both caused by the same pathogen: Chlamydia psittaci, which is an aerobic, Gram-negative bacterium (Figure 11.1). The pathogen originated in South America where it infected parrots and other species of psittacine birds. These birds were popular with the Native Americans in the area, who kept them as pets and used their feathers for decoration, but these practices sickened many people. A monk stationed in Peru rst described the disease in 1615 (see quote earlier). The disease remained unknown in Europe until the 1880s when people started importing parrots from South America for household pets. In 1880, Jakob Ritter, a general practitioner, documented an unusual pneumonia-like disease in Switzerland that involved ve members of his brother’s household and two visitors; three of the seven succumbed. He realized that the family’s newly imported parrots and nches were the source of the disease.