What is Influenza?

Influenza is a viral infection that produces fever, chills, sore throat, muscle aches, and cough that last a week or more.⁽¹⁾ We tend to use the term "flu" to describe any type of respiratory or gastrointestinal illness, such as a cold or diarrhea and vomiting that resemble the symptoms of "flu-like illness" (ILI). However, flu is usually associated with a more severe illness and lasts longer than the common cold, and flu does not normally cause vomiting or diarrhea in adults.⁽²⁾

Influenza viruses are RNA viruses of the Orthomyxoviridae family. Influenza A viruses infect humans, animals and birds; Influenza B and C viruses primarily infect humans, while influenza D infects cattle. According to the World Health Organization (WHO), "The influenza virus undergoes high mutation rates and frequent genetic reassortment (combination and rearrangement of genetic material) leading to variability in the HA (hemagglutinin) and NA (neuraminidase) antigens".⁽³⁾

Influenza A viruses are present in ducks, chickens, pigs, horses, whales and seals. Wild birds are the main natural reservoir of influenza A viruses and often cause asymptomatic or mild infection in birds, but can become virulent in both wild and domestic poultry (chickens and turkeys). Pigs can be infected with swine, human and avian viruses, and sometimes these viruses recombine and create new influenza viruses.^(4-5) Influenza A viruses are divided into subtypes based on two proteins on the surface of the virus and can be further divided into different strains, while influenza B viruses are not divided into subtypes but can be divided into lineages and strains.⁽⁶⁾

Because influenza viruses continually mutate 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 geographic areas or countries. Occasionally, an influenza strain emerges to cause an influenza pandemic that spreads globally and is usually associated with more severe illness and increased mortality.⁽⁷⁾ Historically, influenza pandemics with higher rates of complications and death have involved type A influenza strains, such as the one that caused the 1918-19 influenza pandemic.⁽⁸⁾

Over 70% of all respiratory infections that occur during the "flu season" are not influenza type A or B, as there are many other viruses and bacteria that can cause an "flu-like illness" (ILI) . The symptoms of ILI infection are similar to those of influenza, and only laboratory tests can confirm whether an individual has been infected with influenza or an ILI.^(9-10)

The vast majority of people recover from the flu without complications and develop immunity to future infections with the same or a related flu strain that can prevent symptoms of the disease or make it less severe.⁽¹¹⁾ However, there is a greater risk of serious complications and death for older adults and those with compromised immune systems or who suffer from diabetes, kidney disease, heart disease and other chronic health problems.⁽¹²⁾

Between the 1976/1977 and 2006/2007 influenza seasons, the US Centers for Disease Control (CDC) estimated that, depending on the type of influenza and the strain circulating in a given year, influenza-related deaths in the United States varied from a minimum of 3.000 to a maximum of 49.000.⁽¹³⁾ According to the CDC, between 2010 and 2020, flu-related deaths fluctuated between a low of 12.000 and a high of 52.000.⁽¹⁴⁾ These numbers, however, are only estimates because the CDC does not collect information on flu-related deaths for people over the age of 18, so the exact number remains unknown.⁽¹⁵⁾

Both vaccinated and unvaccinated people can become infected with the influenza virus and shed and transmit it in respiratory secretions⁽¹⁶⁾ and wild-type influenza virus has also been shed and identified in feces.⁽¹⁷⁾ Vaccinated and unvaccinated individuals can transmit influenza to others but be asymptomatic and have no apparent clinical symptoms.^(18-19)

Influenza viruses are transmitted through the air via droplets when infected people cough, sneeze or talk. These droplets can end up in the mouth or nose or be inhaled into the lungs of others close to a person with flu.⁽²⁰⁾ Less frequently, a person can also become infected by touching an object or surface that has the flu virus on it and then touching their nose or mouth.⁽²¹⁾

To avoid transmitting the flu virus - or other types of flu-like respiratory infections - to others, people who know they are sick should stay home until they recover. Frequent hand washing with soap and water can help prevent the spread of flu and other viruses. If soap and water are not available, alcohol-based hand sanitizers can also be used. Eating utensils, plates, linens and other personal items used by those who are ill should not be shared without thorough washing. Frequently touched surfaces should be cleaned and disinfected at home, school and work, especially if used by sick people.⁽²²⁾