Birds which are infected by psittacosis or serve as the pathogen carriers are considered as the source of infection. Currently, more than 140 types of birds are known to contract or carry the pathogen which is mostly found in secretions and feathers. Infections in birds are unapparent and the signs are characteristic. Although most of the infected birds show no or mild symptoms, the pathogens can be excreted for several months. A patient can also become a minor source of infection if he/she excretes pathogens in sputum.

Psittacosis can be transmitted via the respiratory tract. Besides being directly transmitted to humans via droplet, the bacteria can be indirectly transmitted by inhaling an aerosol of infected birds’ feces via the respiratory tract. However, according to the reports, few patients experience the onsets without the contact history of birds. Psittacosis is rarely transmitted via direct person-to-person contact.

Populations are generally susceptible and the occurrence has no significant gender difference. It is an epidemic disease all year round. The infection rate is closely related to the frequency of bird contact: parrot and poultry raisers easily contract the disease. Although certain immunity can be acquired after the infection is cured, it is not strong enough to prevent the repeated onsets and the following infection.

Psittacosis affects many areas around the world. Although severe outbreaks have occurred, few cases have been reported in recent years. Ritter reported the case of human psittacosis in Switzerland in 1879. In the early 1960s, psittacosis was proved to exist in China. The disease usually spread sporadically, while outbreaks and epidemics occur on a small scale sometimes. As people who are often exposed to poultry and birds have a higher infection rate, psittacosis is considered as an occupational disease for poultry workers.

The pathogenesis of psittacosis is still not clear. After being inhaled by humans via the respiratory tract, Cps enters the blood circulation, then attacks the mononuclear macrophage system, and proliferates inside. After the above process, Cps invades the lung and other organs via blood. Thus, human psittacosis may be a respiratory tract infection (RTI) or a general infection mainly with respiratory symptoms. The pulmonary lesions often occur in the hilum of the lung and then spread around, resulting in the lobular pneumonia and interstitial pneumonia. Via microscopic examination, infiltration of the mononuclear lymphocytes can be discovered in the alveolar wall and interstitial tissue; deciduous epithelial cells, fibrous protein, lymphocytes, and few neutrophils can be found in the pulmonary alveoli; the inclusion bodies and pathogens can be seen in the cytoplasm of the macrophage in the exudation of pulmonary alveoli. SARS and lobular pneumonia often occur. In severe cases, necrosis of lung tissue may occur accompanying pleurisy. Beside the lung, relative lesions can be found in other involved organs such as the liver, spleen, kidney, meninges, cardiac muscles, and endocardium.

The symptoms include high fever and headache, accompanying relative bradycardia and enlargement of the spleen. Complications such as myocarditis, endocarditis, and meningitis easily occur. In severe cases, the coma and renal failure may lead to patients’ death.

The illness course is long, with a fever course lasting for 3–4 weeks or even several months. The recurrence rate may reach 20 %.