[1] In humans, after an incubation period of 5–19 days, the disease course ranges from asymptomatic to systemic illness with severe pneumonia.

[citation needed] The second week is more akin to acute bacteremic pneumococcal pneumonia with continuous high fevers, headaches, cough, and dyspnea.

[citation needed] The Chlamydia psittaci bacterium that causes psittacosis can be transmitted by mouth-to-beak contact, or through the airborne inhalation of feather dust, dried faeces, or the respiratory secretions of infected birds.

[6] Blood analysis usually shows a normal white cell count, but marked leukocytosis is occasionally apparent.

Differential diagnosis must be made with typhus, typhoid, and atypical pneumonia by Mycoplasma, Legionella, or Q fever.

Diagnosis involves microbiological cultures from respiratory secretions of patients or serologically with a fourfold or greater increase in antibody titers against C. psittaci in blood samples combined with the probable course of the disease.

[10] From 2002 through 2009, 66 human cases of psittacosis were reported to the Centers for Disease Control and Prevention,[citation needed] and most resulted from exposure to infected pet birds, usually cockatiels, parakeets, and macaws.

Infected birds shed the bacteria through feces and nasal discharges, which can remain infectious for several months.

C. psittaci in birds is often systemic and infections can be inapparent, severe, acute, or chronic with intermittent shedding.

Signs in birds include "inflamed eyes, difficulty in breathing, watery droppings, and green urates.

Septicaemia eventually develops and the bacteria become localized in epithelial cells and macrophages of most organs, conjunctiva, and gastrointestinal tract.

C. psittaci strains are similar in virulence, grow readily in cell culture, have 16S-rRNA genes that differ by <0.8%, and belong to eight known serovars.

[citation needed] C. psittaci serovar A is endemic among psittacine birds and has caused sporadic zoonotic disease in humans, other mammals, and tortoises.