Thursday, November 19, 2009
Google Privacy
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The Privacy Center was created to provide you with easy-to-understand information about our products and policies to help you make more informed choices about which products you use, how to use them, and what information you provide to us. {View More}
The Privacy Center was created to provide you with easy-to-understand information about our products and policies to help you make more informed choices about which products you use, how to use them, and what information you provide to us. {View More}
Saturday, September 5, 2009
Influenza
Influenza, also known as flu, contagious infection primarily of the respiratory tract. Influenza is sometimes referred to as grippe. Influenza is caused by a virus transmitted from one person to another in droplets coughed or sneezed into the air. It is characterized by coldlike symptoms plus chills, fever, headaches, muscle aches, and fatigue. Most people recover completely in about a week. But some people are vulnerable to complications such as bronchitis and pneumonia. This group includes children with asthma, people with heart or lung disease, and the elderly. In the United States, people age 65 and older account for about 90 percent of influenza-associated deaths.
Because influenza is highly contagious and spreads easily, it usually appears as epidemics—that is, outbreaks involving many people. If an outbreak spreads around the world—not uncommon in this age of rapid international travel—it is called a pandemic.
Many millions of people develop the flu each year. In most years less than 1 percent of those infected die. Nonetheless, this translates into large numbers. The United States Centers for Disease Control and Prevention (CDC) estimates that influenza causes more than 20,000 deaths in the United States each year; combined, influenza and pneumonia are among the nation’s ten leading causes of death. During epidemics and pandemics, death rates soar. The influenza pandemic that occurred between 1918 and 1919—the worst on record—killed about 500,000 people in the United States and more than 20 million people worldwide.
Because influenza is highly contagious and spreads easily, it usually appears as epidemics—that is, outbreaks involving many people. If an outbreak spreads around the world—not uncommon in this age of rapid international travel—it is called a pandemic.
Many millions of people develop the flu each year. In most years less than 1 percent of those infected die. Nonetheless, this translates into large numbers. The United States Centers for Disease Control and Prevention (CDC) estimates that influenza causes more than 20,000 deaths in the United States each year; combined, influenza and pneumonia are among the nation’s ten leading causes of death. During epidemics and pandemics, death rates soar. The influenza pandemic that occurred between 1918 and 1919—the worst on record—killed about 500,000 people in the United States and more than 20 million people worldwide.
Influenza Types
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.
Transmission of Influenza
Influenza viruses pass from person to person mainly in droplets expelled during sneezes and coughs. When a person breathes in virus-laden droplets, the hemagglutinin on the surface of the virus binds to enzymes in the mucous membranes that line the respiratory tract. The enzymes, known as proteases, cut the hemagglutinin in two, which enables the virus to gain entry into cells and begin to multiply. These proteases are common in the respiratory and digestive tracts but not elsewhere, which is why the flu causes primarily a respiratory illness with occasional gastrointestinal symptoms. In the 1990s scientists discovered that some flu strains also can use the enzyme plasmin to cut hemagglutinin. Plasmin is common throughout the body, enabling the flu strains to infect a variety of tissues.
Although an influenza epidemic can occur at any time of year, flu season in temperate regions typically begins with the approach of winter—November in the Northern Hemisphere, April in the Southern Hemisphere. Flu viruses spread more easily during cold weather because people tend to spend more time crowded together in homes and schools, as well as buses, subways, and other places with poor ventilation. An epidemic may be restricted to a town or city or may quickly spread geographically as infected people travel aboard motor vehicles, airplanes, and ships.
Although an influenza epidemic can occur at any time of year, flu season in temperate regions typically begins with the approach of winter—November in the Northern Hemisphere, April in the Southern Hemisphere. Flu viruses spread more easily during cold weather because people tend to spend more time crowded together in homes and schools, as well as buses, subways, and other places with poor ventilation. An epidemic may be restricted to a town or city or may quickly spread geographically as infected people travel aboard motor vehicles, airplanes, and ships.
Symptoms and Diagnosis for Influenza
Influenza is an acute disease with a rapid onset and pronounced symptoms. After the influenza virus invades a person’s body, an incubation period of one to two days passes before symptoms appear. Classic symptoms include sore throat, dry cough, stuffed or runny nose, chills, fever with temperatures as high as 39º C (103º F), aching muscles and joints, headache, loss of appetite, occasional nausea and vomiting, and fatigue. For most people flu symptoms begin to subside after two to three days and disappear in seven to ten days. However, coughing and fatigue may persist for two or more weeks.
Death from influenza itself is rare. But influenza can aggravate underlying medical conditions, such as heart or lung disease. Invading influenza viruses produce inflammation in the lining of the respiratory tract, damage that increases the risk that secondary infections will develop. Common complications include bronchitis, sinusitis, and bacterial pneumonia, occurring most frequently in the elderly, people on chemotherapy, and people with acquired immunodeficiency syndrome (AIDS) or another disease that compromises the immune system. If properly treated, these complications seldom are fatal.
Because influenza is so common and exhibits standard symptoms, doctors often diagnose the illness based on the season and whether flu cases have recently been reported in the area. To prove a diagnosis of influenza in a patient, the virus must be isolated from the person’s nasal or cough secretions or blood and identified under a microscope.
Death from influenza itself is rare. But influenza can aggravate underlying medical conditions, such as heart or lung disease. Invading influenza viruses produce inflammation in the lining of the respiratory tract, damage that increases the risk that secondary infections will develop. Common complications include bronchitis, sinusitis, and bacterial pneumonia, occurring most frequently in the elderly, people on chemotherapy, and people with acquired immunodeficiency syndrome (AIDS) or another disease that compromises the immune system. If properly treated, these complications seldom are fatal.
Because influenza is so common and exhibits standard symptoms, doctors often diagnose the illness based on the season and whether flu cases have recently been reported in the area. To prove a diagnosis of influenza in a patient, the virus must be isolated from the person’s nasal or cough secretions or blood and identified under a microscope.
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