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The incubation period is typically seven to ten days, in infants or young children, after which there is usually mild respiratory symptoms, mild coughing, sneezing, or runny nose. This is known as the catarrhal stage. After one to two weeks, the coughing classically develops into uncontrollable fits, each with five to ten forceful coughs, followed by a high-pitched "whoop" sound in younger children, or a gasping sound in older children, as the patient struggles to breathe in afterwards (paroxysmal stage). Fits can occur on their own or can be triggered by yawning, stretching, laughing, eating or yelling; they usually occur in groups, with multiple episodes every hour around the clock. This stage lasts two to eight weeks, or sometimes longer. A gradual transition then occurs to the convalescent stage, which usually lasts one to two weeks. This stage is marked by a decrease in paroxysms of coughing, both in frequency and severity, and a cessation of vomiting. Common complications of the disease include pneumonia, encephalopathy, earache, or seizures. Infection in newborns is particularly severe.