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Content Update: October 2022

Monkeypox (pathophysiology, clinical features, vaccination, treatment, and prevention) is discussed at the end of the chapter, in the section Human Monkeypox Infections.

Content Update: February 2022.

CORONAVIRUS-19 (COVID-19) is now discussed in Chapter 154-B, with most recent update for that chapter in February 2022. Coronavirus discussion is deleted from this chapter.

New information on vaccines for Dengue and Ebola are in the sections of this chapter, Dengue fever is discussed in the section Arboviral Infections, and Ebola is discussed in the section Ebola Virus Disease and Other Hemorrhagic Fevers.

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.


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.


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.


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.


Patients with influenza typically present with an abrupt onset of ...

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