Stress associated with nutritional deficiencies, overcrowding, breeding, egg-laying and prolonged transport may cause birds with a latent infection to shed infectious agents. When shedding occurs, the infected birds excrete the bacteria in the feces and nasal discharges and can remain infective for several months. When a person breathes in Chlamydia psittaci bacteria, the lungs' defense mechanisms attempt to neutralize them. The bacteria that avoid this defense start an infection that varies in severity from a mild flu-like illness to severe pneumonia. Generally, the signs and symptoms appear within four to 15 days after exposure.
Psittacosis is primarily a lung disease but it can involve several organs.
Some reports show that inflammation of the liver, lining of the heart cavity, the heart muscle, and the brain can occur. The course of the disease is variable and it can result in death. However, fatal cases are rare. In mild cases, fever may continue for three weeks or more. For accurate diagnosis of psittacosis, a doctor must know that the person has been exposed to birds and that the suspected birds are infected with Chlamydia psittaci .