The word influenza is derived from the Latin word influentia. Italians in the early 16th century first applied the word influenza to outbreaks of any epidemic disease because they blamed such outbreaks on the influence of heavenly bodies. The first known use of the name specifically for the flu occurred in 1743 when an epidemic swept through Rome and its environs.
Today scientists know that members of the family Orthomyxoviridae, a group of viruses that infect vertebrate animals, cause influenza. The virus consists of an inner core of the genetic material ribonucleic acid (RNA) surrounded by a protein coat and an outer lipid (fatty) envelope from which project spikes of proteins called hemagglutinin and neuraminidase. These proteins act as antigens—that is, they elicit an immune response in the human or other host organism that the virus invades. In addition to their role as antigens, hemagglutinin enables the virus to bind to and invade cells and neuraminidase allows the virus to move among cells.
There are three types of influenza viruses, known as A, B, and C. Type A, the most dangerous, infects a wide variety of mammals and birds. It causes the most cases of the disease in humans and is the type most likely to become epidemic. Type B infects humans and birds, producing a milder disease that can also cause epidemics. Type C apparently infects only humans. It typically produces either a very mild illness indistinguishable from a common cold or no symptoms at all. Type C does not cause epidemics.
Influenza type A and B viruses continually change. Some changes involve a series of genetic mutations that, over a period of time, cause a gradual evolution of the virus. Called antigenic drift, this process accounts for most of the changes in influenza viruses that occur from one year to the next. Other changes, less common but more injurious, involve abrupt changes in the hemagglutinin or neuraminidase. This type of change is called antigenic shift and results in a new subtype of the virus. Type A viruses undergo both kinds of transformations; influenza type B viruses apparently change only by the process of antigenic drift.
Scientists further differentiate virus subtypes into strains, generally named for the geographic area where they were first detected. For example, the strains that caused the most infections during the 2001-2002 flu season in the Northern Hemisphere were type A New Caledonia and Moscow strains and Type B Sichuan strain.
Once a person has been infected by a specific strain of influenza, he or she has built up immunity to that strain in the form of antibodies. The person’s immune system then can recognize the strain’s hemagglutinin or neuraminidase and attack them if they reappear. The antibodies offer some protection against antigenic drifts, but not against antigenic shifts. Thus, because the viruses continually change, they can cause repeated waves of infection, even among people previously infected.
Scientists do not understand exactly what causes antigenic shifts. One leading theory suggests that a human strain and an animal strain recombine to create a new strain. This strain has the ability to infect humans but has antigens on its surface that are unfamiliar to the human immune system.
Today scientists know that members of the family Orthomyxoviridae, a group of viruses that infect vertebrate animals, cause influenza. The virus consists of an inner core of the genetic material ribonucleic acid (RNA) surrounded by a protein coat and an outer lipid (fatty) envelope from which project spikes of proteins called hemagglutinin and neuraminidase. These proteins act as antigens—that is, they elicit an immune response in the human or other host organism that the virus invades. In addition to their role as antigens, hemagglutinin enables the virus to bind to and invade cells and neuraminidase allows the virus to move among cells.
There are three types of influenza viruses, known as A, B, and C. Type A, the most dangerous, infects a wide variety of mammals and birds. It causes the most cases of the disease in humans and is the type most likely to become epidemic. Type B infects humans and birds, producing a milder disease that can also cause epidemics. Type C apparently infects only humans. It typically produces either a very mild illness indistinguishable from a common cold or no symptoms at all. Type C does not cause epidemics.
Influenza type A and B viruses continually change. Some changes involve a series of genetic mutations that, over a period of time, cause a gradual evolution of the virus. Called antigenic drift, this process accounts for most of the changes in influenza viruses that occur from one year to the next. Other changes, less common but more injurious, involve abrupt changes in the hemagglutinin or neuraminidase. This type of change is called antigenic shift and results in a new subtype of the virus. Type A viruses undergo both kinds of transformations; influenza type B viruses apparently change only by the process of antigenic drift.
Scientists further differentiate virus subtypes into strains, generally named for the geographic area where they were first detected. For example, the strains that caused the most infections during the 2001-2002 flu season in the Northern Hemisphere were type A New Caledonia and Moscow strains and Type B Sichuan strain.
Once a person has been infected by a specific strain of influenza, he or she has built up immunity to that strain in the form of antibodies. The person’s immune system then can recognize the strain’s hemagglutinin or neuraminidase and attack them if they reappear. The antibodies offer some protection against antigenic drifts, but not against antigenic shifts. Thus, because the viruses continually change, they can cause repeated waves of infection, even among people previously infected.
Scientists do not understand exactly what causes antigenic shifts. One leading theory suggests that a human strain and an animal strain recombine to create a new strain. This strain has the ability to infect humans but has antigens on its surface that are unfamiliar to the human immune system.
