infectious diseases
Influenza
Epidemiology of Influenza
Influenza causes widespread sporadic illness yearly during fall and winter in temperate climates (seasonal epidemics). Seasonal epidemicsare caused by both influenza A and B viruses; since 1968, most seasonal influenza epidemics have been caused by H3N2 (an influenza A virus). Influenza B viruses may cause milder disease but often cause epidemics with moderate or severe disease, either as the predominant circulating virus or along with influenza A. Most influenza epidemics are caused by a predominant serotype, but different influenza viruses may appear sequentially in one location or may appear simultaneously, with one virus predominating in one location and another virus predominating elsewhere. A weekly surveillance report of seasonal influenza in the US is available at the Centers for Disease Control and Prevention'sFluView. Pandemicsare much less common. There have been 6 major influenza pandemics, typically named after the presumed location of origin: 1889: Russian influenza (H2N2) 1900: Old Hong Kong influenza (H3N8) 1918: Spanish influenza (H1N1) 1957: Asian influenza (H2N2) 1968: Hong Kong influenza (H3N2) 2009: Swine influenza (influenza A [H1N1]pdm09) In 2009–2010 an H1N1 flu pandemic occurred—the virus spread to > 70 countries and to all 50 US states. The majority of the deaths occurred in Mexico. The virus was initially referred to as a swine flu virus, but it is a combination of swine, avian, and human influenza viruses. The infection is not acquired through ingestion of pork and is acquired very rarely by contact with infected pigs. Subsequently, the virus name was standardized to influenza A(H1N1)pdm09 to denote the pandemic and distinguish the virus from seasonal H1N1 strains and the 1918 pandemic H1N1 strain. Since 2009, influenza A(H1N1)pdm09 has circulated as a seasonal influenza. Influenza viruses can be spread by Airborne droplets Person-to-person contact Contact with contaminated items Airborne spread appears to be the most important mechanism. Certain patients are at high risk of complications from influenza: Children<5 years; children < 2 years are at particularly high risk Adults>65 years People with chronic medical disorders (eg, cardiopulmonary disease, diabetes mellitus, renal or hepatic insufficiency, hemoglobinopathies, immuodeficiency) Women in the 2nd or 3rd trimester of pregnancy Patients with disorders that impair handling of respiratory secretions (eg, cognitive dysfunction, neuromuscular disorders, stroke, seizure disorders) (because Reye Syndrome is a risk) Morbidity and mortality in these patients may be due to exacerbation of underlying illness, Acute Hypoxemic Respiratory Failure (AHRF, ARDS) , primary influenza pneumonia, or secondary bacterial pneumonia. 