Parents of children in the Southern Door School District have been informed that a child in the district is being treated for pertussis. Commonly referred to as whooping cough, the ailment is highly contagious and affects the respiratory tract. In a letter issued on Monday, school nurse April Soto cautioned parents to watch their child for mild, cold like symptoms. Soto says, if it appears that a child has the symptoms, a physician and the school district should be contacted. Mild symptoms like a runny nose eventually lead to a persistent cough and finally a very deep cough which can make it difficult to breathe, eat or sleep. Soto suggests all family members be vaccinated for pertussis. She notes that young infants are at highest risk of complications from the disease. The Door County Public Health Department offers the pertussis booster shot free of charge to residents.