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What is Influenza?

Updated October 12, 2023


Influenza is a viral infection that produces fever, chills, sore throat, muscle aches, and cough that lasts a week or more.  People tend to use the term "flu" to describe any kind of respiratory or gastrointestinal illness, such as colds or diarrhea and vomiting that resemble “influenza-like-illness” (ILI) symptoms. But influenza is usually associated with more severe illness and lasts longer than the common cold and, normally, influenza does not cause vomiting or diarrhea in adults. 

Influenza viruses are RNA genome viruses in the Orthomyxoviridae family. Influenza A viruses infect humans, animals and birds and influenza B and C viruses mainly infect humans, while influenza type D infects cattle. According to the World Health Organization (WHO), “influenza virus undergoes high mutation rates and frequent genetic reassortment (combination and rearrangement of genetic material) leading to variability in HA (haemagglutinin) and NA (neuraminidase) antigens.” 

Influenza A viruses are found in ducks, chickens, pigs, horses, whales and seals. Wild birds are the primary natural reservoir for influenza A viruses and often cause asymptomatic or mild infection in birds but can become virulent in both wild and domestic poultry (chickens, turkeys). Pigs can be infected with swine, human and bird (avian) viruses and sometimes those viruses recombine and create new influenza viruses.    Influenza A viruses are divided into subtypes based on two proteins on the surface of the virus and can be further broken down into different strains, while influenza B viruses are not divided into subtypes but can be broken into lineages and strains. 

Because influenza viruses are constantly mutating and there are different strains and subtypes that are more or less prevalent among human populations from year to year, outbreaks and epidemics occur in certain geographical areas or countries. Occasionally, an influenza strain will emerge to cause an influenza pandemic that spreads globally and is usually associated with more severe disease and increased mortality.  Historically, influenza pandemics with higher rates of complications and death have involved type A influenza strains like the one that caused the 1918-19 influenza pandemic. 

Over 70 percent of all respiratory infections that occur during the “flu season” are not type A or type B influenza because there are many other viruses and bacteria that can cause respiratory “influenza-like illness” (ILI). ILI infection symptoms are similar to influenza symptoms and only lab tests can confirm whether an individual has been infected by influenza or an ILI.    

The vast majority of people recover from influenza without any complications and develop immunity to future infection with the same strain or a related influenza strain that may prevent illness symptoms or make illness less severe.  There is, however, an increased risk for serious complications and death for the elderly and those with compromised immune systems or who are suffering from diabetes, kidney dysfunction, heart disease, and other chronic health issues. 

Between the 1976/1977 and 2006/2007 flu seasons, the Centers for Disease Control (CDC) estimated that, depending upon the influenza type and strain circulating in a given year, influenza-related deaths in the United States range between a low of 3,000 and a high of 49,000.  According to the CDC, between 2010 and 2020 influenza-associated deaths have ranged between a low of 12,000 to a high of 52,000.  These numbers, however, are only estimates because the CDC does not collect influenza-related death information for persons 18 years and older so the exact number remains unknown. 

IMPORTANT NOTE: NVIC encourages you to become fully informed about Influenza and the Influenza vaccine by reading all sections in the Table of Contents, which contain many links and resources such as the manufacturer product information inserts, and to speak with one or more trusted health care professionals before making a vaccination decision for yourself or your child. This information is for educational purposes only and is not intended as medical advice.


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