The incubation period is 10 to 21 days. The infectious period starts at the onset of symptoms and lasts for approximately two weeks. The symptoms are divided into three stages. During the second, or paroxysmal, stage, vomiting commonly follows coughing. Severe coughing can cause hemorrhage of the membranes in the nose and eyes.
Mental retardation can result from cerebral hemorrhage, as can spastic paralysis.
Complications from pneumonia may lead to bronchiectasis and emphysema in later life. The third stage, the decline, begins after four weeks. The coughing episodes subside and food intake improves. Recovery may last several months. The antibiotic erythromycin may prevent the onset of paroxysmal coughing if given early in the catarrhal stage.