Symptoms usually develop around 5-10 days after infection, but can take as long as 3 weeks to appear. Infected individuals are most contagious during the first two weeks of symptoms.
During the first 1-2 weeks of symptoms, an individual will experience symptoms resembling the common cold, such as fever, runny nose, and a mild cough. Among infants, apnea (or pauses in breathing) is typical.
When the disease worsens, an individual will experience paroxysmal coughing--many, rapid, violent coughs followed by a "whoop" as the individual struggles to breathe and recover. Vomiting and exhaustion commonly follow these paroxysmal coughs. This stage of the disease can last for 1-6 weeks and has been documented to last as long as 10 weeks. The worse the condition, the more frequent the paroxysms, which would also become more common at night. Among babies, paroxysmal coughing is not very common; instead, during this stage, apnea is more common, becoming evident when the babies turn "blue" or cyanotic.
During recovery, coughing fits can still return, although milder and less common, and may persist for many months.