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INTRODUCTION

Viral infections are among the most common illnesses encountered in the ED. Although many are self-limited, some are life-threatening, have specific treatments, or have public health implications. We review serious viral infections that cause disseminated illness, viruses with a predilection for the neurologic system, and new viral threats.

INFLUENZA

Human influenza infection comes from a large, single-stranded RNA virus in the orthomyxovirus family. Influenza A and influenza B cause most human infections, with influenza A typically being more serious.

EPIDEMIOLOGY

Influenza is highly infectious and transmitted via aerosolized respiratory secretions, large droplets, or fomites. Seasonal influenza has varying severity, typically peaking in the winter months of temperate climates. Seasonal disease causes 250,000 to 500,000 annual deaths worldwide and between 5000 and 50,000 deaths in the United States. Outbreaks spread quickly throughout a community, largely among school-age children. Mortality occurs mostly among the elderly and young infants.

Influenza viruses undergo minor variations (antigenic drift) in their surface antigens that allow the virus to reinfect individuals and reemerge each winter. Occasionally, there are major antigenic changes that increase population susceptibility and lead to a pandemic. The 1918 influenza pandemic killed between 50 million and 100 million people worldwide; the potential for a recurrence with devastation from a novel influenza virus understandably drives anxiety among clinicians and public health personnel. Mortality during pandemics occurs more frequently among healthy adults compared to seasonal influenza, when the infirm and those at the extremes of age are more likely to die. In 2009, a pandemic H1N1 strain emerged in Mexico and followed this pandemic pattern. Highly pathogenic avian strains of influenza emerged from China, including the H5N1 strain in 1997 and, more recently, the H7N9 strain in 2013.

PATHOPHYSIOLOGY

Influenza is transmitted usually by sneezing or coughing and via touching hands. The incubation period is typically 1 to 4 days. Viral replication in upper and lower respiratory tract epithelium leads to cell damage and respiratory symptoms. Most infections resolve within about a week, but viral pneumonia is one of the more common complications that can result in alveolar damage and hypoxemia.

CLINICAL FEATURES

Patients with influenza typically present with an abrupt onset of fever and respiratory symptoms. Most cases are self-limited; however, some patients, particularly the elderly, the very young, and those with comorbid conditions, require hospitalization or die from complications of influenza. Additionally, influenza can lead to exacerbations of chronic medical conditions, such as congestive heart failure or chronic obstructive lung disease. Delirium is common in the elderly.

Pneumonia complicates some influenza infections. Primary influenza pneumonia develops quickly and can progress to acute respiratory distress syndrome, often within 24 hours. Bacterial superinfections also occur. Classically, secondary Staphylococcus aureus pneumonia, including community-acquired methicillin-resistant S. aureus, can follow influenza infection.

DIAGNOSIS

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