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Viral illnesses are among the most common reasons that people come to an emergency department. Although many viral illnesses are self-limited, some can be life-threatening, have specific treatments, or have public health implications. This chapter reviews some of the more common specific viral infections.


Influenza infections occur worldwide with peak activity found in temperate climates between late December and early March. Influenza virus is transmitted by aerosolized or droplet transmission from the respiratory tract of infected persons or by direct contact. After exposure, the incubation period is about 1 to 4 days, and viral shedding begins 24 hours before the onset of symptoms.

Clinical Features

Symptoms of influenza are often abrupt in onset and include fever with chills or rigors, headache, myalgia, and generalized malaise. Additional symptoms may include cough, rhinorrhea, sore throat, and tender, enlarged cervical lymph nodes. Elderly patients may not have classic symptoms and can present with only fever, malaise, confusion, and/or nasal congestion. Almost 50% of affected children have associated gastrointestinal symptoms, but these are unusual in adults. 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.

Respiratory complications of acute infection include primary influenza pneumonitis, secondary bacterial pneumonia, croup, and exacerbation of chronic obstructive pulmonary disease. Avoid aspirin use in patients with suspected or confirmed influenza due to an association with Reye's syndrome. Other uncommon complications include Guillain–Barré syndrome, myocarditis, and pericarditis.

Diagnosis and Differential

A clinical diagnosis of influenza during a known outbreak can be made using history and physical examination. Rapid antigen assays can give results within 15 minutes but have variable sensitivity (10% to 80%) that can result in higher levels of false-negative results when influenza activity is high. Although specificity is high (90% to 95%), false-positive results can also occur, especially when influenza activity is low. Molecular diagnostic tests are available and are more sensitive and specific than antigen tests.

During influenza season, consider diagnostic testing in the outpatient setting for immunocompetent patients at high risk for influenza complications and within 5 days of symptom onset, and in immunocompromised patients regardless of time since illness onset. Many hospitals implement protocols for influenza testing of patients admitted with respiratory disease. Once influenza has been documented in the local community, influenza testing is not typically indicated for otherwise healthy outpatients with signs and symptoms consistent with influenza, especially during the peak of activity.

Emergency Department Care and Disposition

  1. The majority of healthy patients with acute influenza infections will have self-limited, uncomplicated illnesses that can be successfully treated with symptomatic care measures in an outpatient setting.

  2. Patients with more severe systemic symptoms or comorbid conditions may ...

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