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Influenza Virus Definition

Influenza (often referred as "the flu") is a respiratory disease that is caused by a group of viruses. There are three types of influenza viruses, but only two cause pandemic diseases in humans. Types A and B cause the respiratory illnesses that commonly occur during winter months. Type C influenza is typically asymptomatic, but occasionally causes very mild respiratory problems. The vaccinations offered every flu season target types A and B, the strains that cause pandemic illnesses.

  1. Transmission

    • Although influenza is common in winter months, the climate is not directly responsible for its transmission. Influenza is not spread through cold weather. Because of the cold climate, people tend to spend more time indoors staying in contact with others more often. Influenza is spread through respiratory droplets when an individual sneezes or coughs. It can also lie dormant on surfaces. When an individual touches a surface or inhales respiratory droplets of an infected person, the virus is transmitted and incubates for one to two days.

    Structure

    • All three types of influenza viruses make up their own family in the phylogenic tree called orthomyxoviridae. The structure of the viral particles is spherical ranging in diameter of 80 to 120 nanometers. Influenza is an enveloped virus, which means it gets its lipid bilayer from the plasma membrane of host cells. Extending outward from the envelope are spiked antigens made mostly from hemagglutinin, which is the primary attachment protein. Within the center of the structure is the viral genome, eight pieces of single-stranded RNA that integrates into host cells.

    Mutations

    • RNA viruses are the most capable mutating viruses. Every year, new vaccinations are developed to combat the antigenic drift of the influenza virus. The mutated strains are named from their hemagglutinin antigen's new protein structure. Mutations mainly occur to combat and evade the immune system of the host. As influenza mutates, the immune system is unable to recognize the viral invasion and disease progresses.

    Double Infection

    • A special type of antigenic shift occurs in influenza A, when the host is infected with two strains of the virus. When this happens, influenza A is capable of recombination of separate strains creating a new subtype. This is especially dangerous and has led to the commonly known avian and swine flu strains. Since animals like pigs and birds can be reservoirs of influenza, people who handle livestock are commonly infected with these newly mutated strains.

    Influenza Pandemic of 1918

    • Although most people disregard the flu as a dangerous infection, the outbreak of a particular strain named "Spanish Flu" killed more people in the United States than World War I. This 1918 influenza strain killed an estimated 20-40 million people. It infected almost 30 percent of Americans and killed an estimated 675,000 people in the U.S. The deaths from the influenza pandemic of the 1918 were so vast that the average lifespan of humans was decreased by ten years.

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