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 a portion of 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. These outbreaks are usually termed epidemics. 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 or Spanish influenza) 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 (first termed novel H1N1 influenza A swine flu, often later 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. Fortunately, there was a worldwide response that included vaccine production, good hygiene practices (especially hand washing) were emphasized, and the virus (H1N1) caused far less morbidity and mortality than was expected and predicted. The WHO declared the pandemic?s end on Aug. 10, 2010, because it no longer fit into the WHO?s criteria for a pandemic.

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.

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. In 2009, almost all individuals had no antibodies that could recognize the novel H1N1 virus immediately.

Type A viruses are divided into subtypes or strains based on differences in two viral surface proteins called the hemagglutinin (H) and the neuraminidase (N). There are at least 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.

The 2009 pandemic-causing H1N1 virus is a classic example of antigenic shift. 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 (also termed avian 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. A diagram that illustrates both antigenic shift and drift can be found below (see Figure 2) and features influenza A types H1N1 and bird flu, but almost every influenza A viral strain can go through these processes that changes the viral RNA.

Typical clinical features of influenza may include

fever (usually 100 F-103 F in adults and often even higher in children),chills,respiratory symptoms such as cough (more often in adults),sore throat (more often in adults),runny or stuffy nose (especially in children),headache,muscle aches,fatigue, sometimes extreme.

Although nausea, vomiting, and diarrhea can sometimes accompany influenza infection, especially in children, gastrointestinal symptoms are rarely prominent. The term ?stomach flu? is a misnomer that is sometimes used to describe gastrointestinal illnesses caused by other microorganisms. H1N1 infections, however, have caused more nausea, vomiting, and diarrhea than the conventional (seasonal) flu viruses.

Most people who get the flu recover completely in one to two weeks, but some people develop serious and potentially life-threatening medical complications, such as pneumonia. In an average year, influenza is associated with about 36,000 deaths nationwide and many more hospitalizations. Flu-related complications can occur at any age; however, the elderly and people with chronic health problems are much more likely to develop serious complications after the conventional influenza infections than are younger, healthier people. However, the H1N1 virus had developed a different pattern of infection. Unfortunately, the pattern of infection is similar to that of the 1918 ?Spanish flu? pandemic in which young people (pregnant individuals, infants, teens, and adults through age 49) are the most susceptible populations worldwide. Analysis of the people who were likely to develop complications from the H1N1 infection showed that other groups of people were also susceptible, including American Indians, patients with COPD, and obese individuals.

Unfortunately, people may be contagious about 24-48 hours before symptoms appear and, for those people who spontaneously recover, they may shed contagious viruses for about a week.

The flu is presumptively diagnosed clinically by the patient?s history of association with people known to have the disease and their symptoms listed above. Usually, a quick test (for example, nasopharyngeal swab sample) is done to see if the patient is infected with influenza A or B virus. Most of the tests can distinguish between A and B types. The test can be negative (no flu infection) or positive for types A or B. If it is positive for type A, the person could have a conventional flu strain or a potentially more aggressive strain such as H1N1. However, a new test developed by the CDC and a commercial company reportedly can detect H1N1 reliably in about one hour; the test was formerly only available to the military. In 2010, the FDA approved a commercially available test that could detect H1N1 within four hours. Most of the rapid tests are based on PCR technology.

Swine flu (H1N1) and other influenza strains like bird flu are definitively diagnosed by identifying the particular antigens associated with the virus strain. In general, this testing is done in a specialized laboratory and is not done by many doctors? offices or hospital laboratories unless they have purchased the newest test systems. However, doctors? offices are able to send specimens to specialized laboratories if necessary. Because of the large number of H1N1 swine flu cases that occurred in the 2009-2010 and 2010-2011 flu seasons, the vast majority of flu cases (about 95%-99%) were H1N1 flu viruses, the CDC has recommended only hospitalized patients? flu virus strains be sent to reference labs to be identified.

Flu vaccine

Most of the illness and death caused by influenza can be prevented by annual influenza vaccination. The CDC?s current Advisory Committee on Immunization Practices (ACIP) issued recommendations for everyone 6 months of age and older, who do not have any contraindications to vaccination, to receive a flu vaccine each year.

Flu vaccine (influenza vaccine made from inactivated and sometimes attenuated [noninfective] virus) is specifically recommended for those who are at high risk for developing serious complications as a result of influenza infection.

A new vaccine type, Fluzone Intradermal, was approved by the FDA in 2011 (for adults 18-64 years of age). This injection goes only into the intradermal area of the skin, not into the muscle (IM) like most conventional flu shots and uses a much smaller needle than the conventional shots. This killed viral preparation is supposed to be about as effective as the IM shot but claims to produce less pain and fewer side effects.?Medicinenet.com

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