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Influenza in Adult
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Influenza in Adult

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Contributors: Susan Voci MD, Sumanth Rajagopal MD, William Bonnez MD
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Influenza (flu) is a universally common epidemic illness caused by several subtypes of type A or by type B influenza virus. Type C influenza virus exists and produces a mild respiratory illness but is not believed to cause epidemics. The most common subtypes of influenza A are H1N1 and H3N2. A subtype of avian influenza A that emerged in Asia (H5N1) produces infrequent but often fatal human illness. A swine-origin influenza A (H1N1) virus (swine flu) outbreak, beginning in March and April of 2009, reinforces that interspecies transmission of human and animal influenza A viruses and subsequent reassortment are possible and are, in fact, occurring, resulting in new viruses. Unlike previous documented cases of swine influenza A/H1N1 infection in humans, which occurred only in persons exposed to pigs or to a family member with exposure to pigs, the 2009 virus spread easily between humans with no pig exposure. Studies showed that this virus had pig as well as avian and human genes.

Influenza is highly contagious and is spread by aerosol droplets. The incubation period is 1-4 days, but it becomes contagious 1 day prior to the onset of symptoms. The mortality rate of influenza is low but tends to be higher in the elderly and the immunocompromised. Flu activity usually peaks in February.

Influenza presents with classic flu-like illness consisting of the sudden onset of fever, malaise, sore throat, nonproductive cough, myalgias, headache, and nasal congestion. Chills are common, as are nausea and vomiting in children. After 48 hours, cough may increase and produce sputum. There may be associated dyspnea and/or mild to moderate pleuritic chest pain. Upon physical exam, unilateral or bilateral inspiratory rales may be appreciated or diminished breath sounds. Pregnant women are at increased risk for severe illness from influenza.

Most viruses that affect the respiratory tract can cause a rash, flu included (see viral exanthem).

The most common pulmonary complication of influenza is secondary bacterial pneumonia. This diagnosis can be determined by patient history. This can occur up to 2 weeks after the initial symptoms and includes recurrence of fever, chills, pleuritic chest pain, and productive cough. Many bacteria may be the culprit, but the most common are pneumococci. Staphylococcus aureus has also been implicated in children; there has been an increase in the number of deaths in which both influenza and pneumonia or bacteremia due to S aureus were identified.

Primary viral pneumonia is the complication responsible for the most influenza-related deaths. Those with pre-existing cardiopulmonary disease or who are pregnant are at the greatest risk. The initial clinical presentation is the same, but dyspnea increases in severity. Productive cough may be blood-tinged. Massive hemoptysis has been reported. When severe, there may be profound respiratory distress with tachypnea, tachycardia, and cyanosis. Rales and wheezes will spread throughout the chest from the lower lung.

The Spanish flu pandemic of 1918 was particularly virulent, killing over 20 million people worldwide. With present day biotechnology, it would be possible to produce an influenza virus weapon with traits of both the H5N1 and the 1918 influenza viruses. As a weapon, influenza could be released as an aerosol.

Current Flu Season
Per the Centers for Disease Control and Prevention (CDC), as of March 28, 2020, laboratory confirmed flu activity as reported by clinical laboratories continues to decrease sharply and is now low. Influenza-like illness activity is still elevated. Note that the COVID-19 pandemic is affecting health care-seeking behavior.

Influenza severity indicators remain moderate to low overall, but hospitalization rates differ by age group, with high rates among children and young adults. In the United States, influenza A(H1N1)pdm09 viruses are now the most commonly reported influenza viruses this season. Previously, influenza B/Victoria viruses predominated nationally.


J10.1 – Influenza due to other identified influenza virus with other respiratory manifestations

6142004 – Influenza

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The differential includes other viral respiratory diseases that present with a flu-like illness. The season and local occurrences of influenza should be considered.

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Last Updated: 04/03/2020
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Influenza in Adult
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Influenza (Young child) : Chills, Cough, Diarrhea, Fever, Vomiting, Nasal congestion, Myalgia, Pharyngitis
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