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 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 (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.


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.


Typical clinical features of influenza include fever (usually 100 F-103 F in adults and often even higher in children), respiratory symptoms such as: 


  • cough, 
  • sore throat, 
  • runny or stuffy nose,
  • 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. Novel 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 novel 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.


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 types 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, cancerpatients, 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 calledantigenic "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 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.


What are some treatments an individual can do at home for the flu?


First, individuals should be sure they are not members of a high-risk group that is more susceptible to getting severe flu symptoms. Check with your physician if you are unsure if you are a higher-risk person. Home care is recommended by the CDC if a person is normally healthy with no underlying diseases or conditions (for example, asthma, lung disease, pregnant, or immunosuppressed).


Increasing liquid intake, warm showers, and warm compresses, especially in the nasal area, can reduce the body aches and reduce nasal congestion. Nasal strips and humidifiers may help reduce congestion, especially while trying to sleep. Some physicians recommend nasal irrigation with saline to further reduces congestion; some recommend nonprescription decongestants.


Fever can be treated with over-the counteracetaminophen (Tylenol) or ibuprofen (Motrin and others) (read labels for safe dosage). Cough can be suppressed by cough drops and over-the-counter cough syrup. If an individual's symptoms at home get worse, their doctor should be notified.


When should a person go to the emergency department for the flu?


The CDC has recently published guidelines on who should go to the emergency department for flu symptoms and who should not go. The CDC published these guidelines to avoid a crush of people going to the emergency department during the H1N1 flu pandemic and utilizing limited resources needed for true emergency patients such as cardiac or trauma patients and to avoid transmitting the virus to high-risk patients.


The CDC guidelines are as follows for children and adults. The CDC urges normally healthy people who get the flu to stay home as the large majority of infected individuals will recover without antiviral medications or other treatments, and staying home should limit the viral spread.


The CDC urges people to seek emergency medical care for a sick child with any of these symptoms:


  1. Fast breathing or trouble breathing.
  2. Bluish or gray skin color.
  3. Not drinking enough fluids.
  4. Severe or persistent vomiting.
  5. Not waking up or not interacting.
  6. Being so irritable that the child does not want to be held.
  7. Flu-like symptoms improve but then return with fever and cough.


The following is the CDC's list of symptoms that should trigger emergency medical care for adults:


  1. Difficulty breathing or shortness of breath.
  2. Pain or pressure in the chest or abdomen.
  3. Sudden dizziness.
  4. Confusion.
  5. Severe or persistent vomiting.
  6. Flu-like symptoms improve but then return with fever and worse cough.
  7. Having a high fever for more than three days is another danger sign, according to the WHO, so the CDC has also included this as another serious symptom.
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