Influenza overview
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Alejandro Lemor, M.D. [2]
Overview
Historical Perspective
Classification
Influenza virus can be classified into 3 main types: A, B and C. Based on the surface glycoproteins, hemagglutinin (HA) and neuraminidase (NA), influenza type A is subclassified into several subtypes and can infect many species, including humans. Type B can only infect humans and type C is not that common and causes mild illness.
Pathophysiology
Influenza virus is under constant evolutionary change. These genetic changes may be small and continuous or large and abrupt. Small genetic changes happen continuously in Type A and Type B influenza as the virus makes copies of itself. This process is called antigenic drift. Drifting happens frequently enough to make new strains of virus unrecognizable to the human immune system. Type A influenza also undergoes infrequent and sudden changes known as antigenic shift. Antigenic shift occurs when two different flu strains infect the same cell and combine portions of their genetic material. The novel assortment of HA and/or NA proteins in a shifted virus may create a new influenza A subtype. Influenza viruses spread mainly through tiny droplets expelled when people with the disease cough, sneeze, or talk. These droplets can land in the mouths or noses of people who are nearby or be inhaled into the lungs.
Causes
Differentiating Influenza from other Diseases
Influenza should be differentiated from other conditions that cause fever, cough, myalgias, headache and fatigue, such as RSV infection, dengue fever, parainfluenza virus infection, ebola infection adenovirus infection, rhinovirus infection, and legionella infection.
Epidemiology and Demographics
The CDC estimates that 5 to 20 percent of Americans are affected by the flu during each flu season, which typically lasts from October to March. Children are 2 to 3 times more likely than adults to get sick with the flu, and children frequently spread the virus to others. For elderly people, newborn babies, pregnant women, and people with certain chronic illnesses, the flu and its complications can be life-threatening. Although most people recover from the illness, between 3,000 and 49,000 US citizens die from the flu and its complications every year.[1]