After a 1 to 3 week incubation period, onset may be insidious or abrupt, with fever, chills, malaise, and anorexia. The temperature rises gradually rises, and cough develops, initially dry but at times mucopurulent.
- During the second week, pneumonia and frank consolidation may occur with secondary purulent lung infection. The temperature remains elevated for 2 to 3 weeks then falls slowly. The course may be mild or severe, depending on the patient's age and the extent of pneumonia.