Respiratory syncytial virus is the most common cause of bronchiolitis and pneumonia in children younger than 1 year. Outbreaks occur seasonally in winter and early spring. Infection does not result in lasting immunity, and reinfection can occur. Coronavirus causes the common cold (nasal obstruction and discharge, sneezing, no fever or mild fever, occasional sore throat, and/or cough) and acute pharyngitis (sore throat, with or without cervical adenopathy, ulceration, and conjunctivitis). Influenza type A is the leading cause of influenza. Influenza is a systemic illness characterized by the sudden onset of fever, headache, myalgias, malaise, and prostration, followed by cough, nasal obstruction, and sore throat. The lower respiratory tract may also be involved. Parainfluenza viruses are the leading cause of croup, or acute laryngotracheobronchitis, in children. This infection involves both the upper and lower respiratory tracts. Inflammation in the subglottic area leads to hoarseness, dyspnea, a barking cough, and inspiratory stridor. Ref: Levinson W. (2012). Chapter 39. RNA-Enveloped Viruses. In W. Levinson (Ed),Review of Medical Microbiology & Immunology, 12e.