Pertussis is a highly contagious bacterial disease of the respiratory tract, caused by Bordetella pertussis. It occurs mainly in infants and young children, and is easily transmitted from person to person, mainly through droplets. The first symptoms generally appear 7–10 days after infection, and include mild fever, runny nose, and cough, which in typical cases gradually develops into a paroxysmal cough followed by whooping (hence the common name of whooping cough).In the youngest infants, the paroxysms may be followed by periods of apnoea. Pneumonia is a relatively common complication; seizures and encephalopathy occur more rarely. Untreated patients may be contagious for three weeks or more following onset of the cough. Pertussis can be prevented by immunization.