Acute viral respiratory illnesses account for ≥50% of all acute illnesses; adults have 3-4 cases per person per year. The clinical presentations of viral infections generally are not specific enough to allow an etiologic diagnosis, and viral illnesses are typically grouped into clinical syndromes (e.g., the “common cold,” pharyngitis, tracheitis, pneumonia). This section will cover the six major groups of respiratory viruses; see Table 101-2 for an overview and Chap. 58. Sore Throat, Earache, and Upper Respiratory Symptoms for additional details on viral respiratory infections.