Psittacine birds, pigeons, and turkeys may harbor C. psittaci in their blood, feathers, tissue, nasal secretions, liver, spleen, and feces. Transmission to humans occurs primarily through inhalation of dust from bird droppings that contain C. psittaci; less commonly, it results from direct contact with infected secretions or body tissue, as in laboratory personnel who work with birds. Psittacosis is an occupational disease of zoo and pet-shop employees or visitors, poultry farmers, and poultry ranchers. People may acquire the disease by handling sick birds or through mouth-to-beak resuscitation. Person-to-person transmission can happen, but it is rare and can result in severe infection. Infection is thought to develop in the respiratory system after organisms are inhaled from aerosolized dried avian excreta or respiratory secretions from sick birds. C. psittaci then attaches to the respiratory epithelial cells. After the initial inoculation, the organism spreads via the bloodstream to the reticuloendothelial system.

After an incubation period of 4 to 15 days, onset of symptoms may be insidious or sudden. Clinical effects include chills and a low-grade fever that increases to 103° to 105° F (39.4° to 40.6° C) for 7 to 10 days then declines, with treatment, during the second or third week. Other signs and symptoms include headache, myalgia, sore throat, cough (may be dry, hacking, and nonproductive or may produce blood-tinged sputum), abdominal distention and tenderness, nausea, vomiting, photophobia, decreased pulse rate, slightly increased respiratory rate, secondary purulent lung infection, and a faint macular rash called Horder spots, which resemble the rose spots observed in typhoid fever but appear on the face. Patients may also develop erythema multiforme and erythema nodosum. Epistaxis is common. Renal symptoms include acute glomerulonephritis and tubulointerstitial nephritis. Musculoskeletal symptoms include reactive arthritis that is usually polyarticular and, rarely, rhabdomyolysis. Severe infection also produces delirium, stupor, extensive pulmonary infiltration with cyanosis, acute respiratory failure, sepsis, and septic shock. Psittacosis may recur but is usually milder.