The virus causes classic influenza syndrome.
Incubation period is short (1–3 days). The classic influenza syndrome is a febrile illness of sudden onset, characterized by tracheitis and marked myalgias. Headache, chills, fever, mal-aise, myalgias, anorexia, and sore throat appear suddenly. The body temperature rapidly rises to 101–104°F (38.3–40.0°C) and respiratory symptoms ensue. Nonproductive cough is characteristic.
Sneezing, rhinorrhea, and nasal obstruction are common. Patients may also report photophobia, hoarseness, nausea, vomiting, diarrhea, and abdominal pain. They appear acutely ill and are usually coughing. Minimal to moderate nasal obstruc-tion, nasal discharge, and pharyngeal inflammation may be present. Lung examination is usually normal.
The viruses mostly cause subclinical infections. They do not display the classic syndrome described above. Moreover, the influenza syndrome is uncommon in children and is not seen in infants. A given patient may exhibit symptoms including pre-dominantly sneezing, nasal obstruction, and nasal discharge (common cold); nasal obstruction, discharge, and sore throat (upper respiratory illness); sore throat with erythema (phar-yngitis); hoarseness (laryngitis); or cough (tracheobronchitis). Fever may be absent.