Classic flu symptoms are abrupt in onset and include fever of 38.6°C to 39.8°C (101°F to 103°F) with chills or rigor, headache, myalgia, and generalized malaise. Respiratory symptoms include dry cough, rhinorrhea, and sore throat, frequently with bilateral tender, enlarged cervical lymph nodes. The elderly usually do not have classic symptoms and may present with only fever, malaise, confusion, and nasal congestion. Almost 50% of affected children have gastrointestinal symptoms, but these are unusual in adults. The fever generally lasts 2 to 4 days, followed by recovery from most of the systemic symptoms within 3 to 7 days. Cough and malaise may persist for several weeks.
Common respiratory complications of acute influenza infection include primary influenza pneumonitis, secondary bacterial pneumonia, croup, and exacerbation of chronic obstructive pulmonary disease. The presence of dyspnea and hypoxia should raise the suspicion for pulmonary involvement. Aspirin therapy should not be used in patients with suspected or confirmed influenza due to the association with Reye syndrome. Other rare complications include Guillain-Barré syndrome, myocarditis, and pericarditis.