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Harrison's Principles of Internal Medicine, 18e
| Part 8. Infectious Diseases > Section 13. Infections Due to DNA and RNA Respiratory Viruses > |
Chapter 187. Influenza
Sections: Influenza: Introduction, Further Readings.
Topics Discussed: influenza; respiratory tract infections, upper.
Excerpt:"Influenza is an acute respiratory illness caused by infection with influenza viruses. The illness affects the upper and/or lower respiratory tract and is often accompanied by systemic signs and symptoms such as fever, headache, myalgia, and weakness. Outbreaks of illness of variable extent and severity occur nearly every year. Such outbreaks result in significant morbidity rates in the general population and in increased mortality rates among certain high-risk patients, mainly as a result of pulmonary complications.The most extensive and severe outbreaks of influenza are caused by influenza A viruses, in part because of the remarkable propensity of the H and N antigens of these viruses to undergo periodic antigenic variation. Major antigenic variations, called antigenic shifts, are seen only with influenza A viruses and may be associated with pandemics. Minor variations are called antigenic drifts. Antigenic variation may involve the hemagglutinin alone or both the hemagglutinin and the neuraminidase. An example of an antigenic shift involving both the hemagglutinin and the neuraminidase is that of 1957, when the predominant influenza A virus subtype shifted from H1N1 to H2N2; this shift resulted in a severe pandemic, with an estimated 70,000 excess..."
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