Flu
(Influenza, Conventional and H1N1)
Medical Author: Charles Davis, MD, PhD
Medical Editor: Melissa Conrad Stöppler, MD
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What to Do if You Get the Flu
Medical Author: Melissa Conrad Stöppler, MD
Medical Editor: William C. Shiel Jr., MD, FACP, FACR
 Influenza, or the "flu," is a illness of the breathing
system (respiratory system) and muscles caused by a virus. While a vaccine is
available to prevent the flu, its effectiveness varies according to the degree
of match between the viral strains used to prepare the vaccine and those strains
actually in circulation in a given year. Not everyone receives the flu vaccine, and even
some of those who do can develop symptoms of the flu.
Mild cases of the flu may seem like common colds. But most cases of the flu
can be distinguished from colds because the symptoms (cough, muscle aches and
pains, sore throat, fatigue, and headache)
are more severe than those of the common cold. Flu symptoms also tend to occur suddenly and include high fevers
(temperatures of 101 degrees F or more). In children, fevers are typically even
higher than those in adults.
The flu is a serious illness that can be fatal in people
whose immune systems are weakened, the elderly, and those with chronic medical
conditions. Even healthy people who develop the flu cannot work, attend school,
or participate in normal activities for several days. Complications of the flu
can develop in anyone and include pneumonia, ear infections, sinus infections,
or bronchitis.
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What is influenza?
Influenza, commonly called "the flu," is an illness caused by RNA viruses that infect the respiratory tract of many animals, birds, and humans. In most people, the infection results in the person getting fever, cough, headache, and malaise (tired, no energy); some people also may develop a sore throat, nausea, vomiting, and diarrhea. The majority of individuals has symptoms for about one to two weeks and then recovers with no problems. However, compared with most other viral respiratory infections, such as the common cold, influenza (flu) infection can cause a more severe illness with a mortality rate (death rate) of about 0.1% of people who are infected with the virus.
The above is the usual situation for the yearly occurring "conventional" or "seasonal" flu strains. However, there are situations in which some flu outbreaks are severe. These severe outbreaks occur when the human population is exposed to a flu strain against which the population has little or no immunity because the virus has become altered in a significant way. Unusually severe worldwide outbreaks (pandemics) have occurred several times in the last hundred years since influenza virus was identified in 1933. By an examination of preserved tissue, the worst influenza pandemic (also termed the Spanish flu) occurred in 1918 when the virus caused between 40-100 million deaths worldwide, with a mortality rate estimated to range from 2%-20%.
In April 2009, a new influenza strain against which the world population has little or no immunity was isolated from humans in Mexico. It quickly spread throughout the world so fast that the WHO declared this new flu strain (termed novel H1N1
influenza A swine flu, often shortened to H1N1 or swine flu) as the cause of a pandemic on June 11, 2009. This was the first declared flu pandemic in 41 years.
Haemophilus influenzae is a bacterium that was incorrectly considered to cause the flu until the virus was demonstrated to be the correct cause in 1933. This bacterium can cause lung infections in infants and children, and it occasionally causes ear, eye, sinus, joint, and a few other infections,
but it does not cause the flu.
What are the causes of the flu?
The flu (influenza) viruses
Influenza viruses cause the flu and are divided into three types, designated A, B, and C. Influenza types A and B are responsible for epidemics of respiratory illness that occur almost every winter and are often associated with increased rates of hospitalization and death. Influenza type C differs from types A and B in some important ways. Type C infection usually causes either a very mild respiratory illness or no symptoms at all; it does not cause epidemics and does not have the severe public-health impact of influenza types A and B. Efforts to control the impact of influenza are aimed at types A and B, and the remainder of this discussion will be devoted only to these two types.
Influenza viruses continually change over time, usually by mutation (change in the viral RNA). This constant changing often enables the virus to evade the immune system of the host (humans, birds, and other animals) so that the host is susceptible to changing influenza virus infections throughout life. This process works as follows: a host infected with influenza virus develops antibodies against that virus; as the virus changes, the "first" antibody no longer recognizes the "newer" virus and infection can occur because the host does not recognize the new flu virus as a problem until the infection is well under way. The first antibody developed may, in some instances, provide partial protection against infection with a new influenza virus. Unfortunately, almost all individuals have no antibodies that will recognize the novel H1N1 virus immediately. Consequently, without vaccination, the majority of the human population is susceptible to novel H1N1 flu.
Type A viruses are divided into types based on differences in two viral surface
proteins called the hemagglutinin (H) and the neuraminidase (N). There are 16
known H subtypes and nine known N subtypes. These surface proteins can occur in
many combinations. When spread by droplets or direct contact, the virus, if not
killed by the host's immune system, replicates in the respiratory tract and
damages host cells. In people who are immune compromised (for example, pregnant individuals, infants, cancer patients, asthma patients, people with pulmonary disease and many others), the virus can cause viral pneumonia or stress the individual's system to make them more susceptible to bacterial infections, especially bacterial pneumonia. Both pneumonia types, viral and bacterial, can cause severe disease and sometimes death.
Antigenic shift and drift
Influenza type A viruses undergo two kinds of changes. One is a series
of mutations that occurs over time and causes a gradual evolution of the
virus. This is called antigenic "drift." The other kind of
change is an abrupt change in the hemagglutinin and/or the neuraminidase
proteins. This is called antigenic "shift." In this case, a new
subtype of the virus suddenly emerges. Type A viruses undergo both kinds
of changes; influenza type B viruses change only by the more gradual
process of antigenic drift and therefore do not cause pandemics.
A diagram that illustrates both antigenic shift and drift can be found at http://www.medicinenet.com/swine_flu/page5.htm and shows how both mechanisms can lead to antigenically diverse virus strains. The U.S. Centers for Disease Control and Prevention (CDC) has indicated that novel H1N1 swine flu has an RNA genome that contains five RNA strands derived from various swine flu strains, two RNA strands from bird flu strains, and only one RNA strand from human flu strains. They suggest mainly antigenic shifts over about 20 years have led to the development of novel H1N1 flu virus.
Next: What are flu symptoms? »
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