Influenza is caused by influenza viruses types A, B, and C. Type A is the most virulent in humans and causes pandemics through antigenic drift or shift. It commonly infects the respiratory tract causing symptoms like cough, fever and sore throat. Treatment involves antivirals, supportive care and antibiotics for secondary infections. Parainfluenza and respiratory syncytial virus are other common respiratory viruses that often infect children and cause croup or pneumonia.