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What is influenza? – Facts and Fallacies

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What is influenza?

If you are wondering – what is influenza – the answer is straightforward. It is a disease of the respiratory tract resulting from infection by the influenza virus. Those outside of the medical field refer to it as flu. It afflicts 5% to 20% of residents of the United States and results in 36,000 deaths each year.  It is the cause of 200,000 hospitalizations.  It is also a worldwide disease. There are many facts about it that the average person should know but doesn’t. There are also many fallacies about it.

There are three types of viruses that cause flu. It produces some typical signs and symptoms. It is highly contagious and spreads mostly during flu season. It is usually self-limited but can be deadly and/or associated with complications. There is specific treatment when the risk of death or complications is increased. Vaccines can be but are not always preventative.

Types of Influenza viruses

The three types of influenza viruses are A, B and C. All types have two different kinds of what is influenzaproteins (antigens) on their surfaces. They are hemagglutinin (HA) and neuraminidase (NA). At the time of this post there are 18 different types of HA proteins and 11 different types of NA proteins. There are various subtypes of the influenza virus based on the different combinations of these proteins. Thus, at the present time there are 198 (18 X 11) subtypes. New subtypes develop through the process of antigenic shift. The emergence of a new subtype that infects humans can result in a pandemic.

Wild birds are the primary natural reservoirs of all the subtypes, but each subtype has its own host range. The host range for the current group of influenza A virus subtypes is one of or a combination of the following:

  • People
  • Poultry
  • Pigs
  • Bats and other animals including horses, whales and seals

The ability of the virus to cause disease though is subtype-specific. H1N1 and H3N2 are the subtypes that currently cause infection in humans. The H2N2 (Asian flu) subtype caused human infections in the past. But there have been no reported cases of it in the last 43 years – thanks to population immunity against it. It remains to be discovered what the next H?N? subtype will be.

There are different strains of A virus subtypes. They are the result of ongoing mutations. The process by which these mutations occur is antigenic drift.

There are no subtypes of the influenza B virus. The reason might be it only causes infection in people. It also mutates at a much slower rate than the A virus. Therefore, its classification is limited to strain and lineage. It usually produces symptoms less severe than the type A flu virus but can still result in significant illness. Unlike the type A virus, it does not cause pandemics.

The influenza C virus also infects only people and does not result in significant illness compared to the type A and B viruses. It does not cause pandemics or epidemics and the mild disease it produces is not limited to the flu season.  

When is the flu season?

It is an annual recurring time period when influenza in humans is most prevalent. In areas of the world between the Arctic Circle and the tropics (temperate zones) it usually begins in the late fall and peaks in the mid-to late winter. In the United States it runs from fall through the early spring. More specifically according to the CDC, it can begin as early as October and last as late as March. Based on CDC data from 1982 through 2014 the peak activity in the United States is between December and February. The greatest number of cases during that same period of time was in the month of February. Cases can occur as late as May though. The seasonal occurrence of influenza varies in the tropics. Seasonal outbreaks occur in some areas. Yet, in other areas, there is a uniform occurrence of cases throughout the year.  

Flu symptoms

The cardinal symptoms of influenza are the following:

  • Chills
  • Fever
  • Fatigue
  • Muscle aches
  • Headaches
  • Dry cough
  • Runny nose (more common in children than adults)

Nausea, vomiting and diarrhea are rare symptoms with influenza since it is an infection of the respiratory tract instead of the gastrointestinal tract. They do occasionally occur though, but mainly in children.

The above symptoms can be present with other upper respiratory tract illnesses caused by different viruses. The most notable one is the common cold. Flu-like symptoms can also be present with some diseases that are not of an infectious nature at all.  Some of the autoimmune diseases are cases in point.

Patients commonly misdiagnose themselves as having the flu when in fact they have a cold. Although both illnesses can share some of the same symptoms there are some notable differences.

  • Fatigue, muscle aches, and fever develop suddenly with influenza and are more intense than with a cold.
  • Fatigue, muscle aches, and fever are more frequent with influenza compared to a cold.
  • Cold symptoms tend to be restricted to the head and nose. Therefore, sneezing, runny nose and stuffy nose are usually present more commonly with a cold than the flu.
  • Sore throat is more common with the cold than with influenza.
  • Headaches tend to be worse if present with the flu than with a cold.  

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The stomach flu misnomer 

Another area of misunderstanding relates to the presence of gastrointestinal symptoms with fever in the absence of respiratory symptoms. Many, including some in the health care field, use the term stomach flu as a diagnosis. The terminology is not appropriate though because the illness is usually gastroenteritis caused by a totally different pathogen than the influenza virus.

The 24-hour flu

The term 24-hour flu is a spinoff of the stomach flu misnomer. When the term is used it also refers to gastroenteritis. Attendant with its use is the notion of 24-hour flu symptoms. They are the nausea, vomiting, diarrhea, headache, chills and fatigue that frequently accompany the gastrointestinal illness. Some forms of gastroenteritis can be associated with symptoms lasting for only 24 hours. But other forms, depending upon the cause, may result in symptoms for as long as 10 days. Thus, the term is not only oftentimes incorrect. It is also a contradiction in terms and a source of confusion.

The use of either term – stomach flu or 24-hour flu is not only a semantic discrepancy. It is also a quagmire for proper disease reporting and medical billing. The reason is the terms don’t exist in the ICD coding manuals. 

Signs of the flu

Signs of the flu are observations that health care professionals or others can make that serve as clues that a person has the flu. Other than fever and perhaps a rapid heart rate, the telltale sign in a person with the cardinal symptoms is the tendency to spend a great deal of time in bed. A person with a common cold is oftentimes up and about and is even able to work. But someone with influenza frequently does not feel like getting out of bed. 

How long does the flu last?

Flu symptoms in most people generally lasts anywhere from three days to two weeks. Symptoms lasting longer than two weeks might be indicative of a flu complication. The use of antiviral drugs can shorten the symptoms in select patients if begun within 48 hours of the onset of symptoms.  

How is the flu spread?

According to most experts, the influenza virus spreads mainly through droplets produced when people sneeze, cough orhow is the flu spread talk. The droplets containing the virus can enter a person’s respiratory tract directly. But more commonly they gain entry by the person touching a contaminated surface, then making contact with the face. The virus then enters through the nose or mouth.  

Incubation period for flu 

The incubation period for an infectious disease is the time between exposure to the pathogen that causes it and the first appearance of signs and symptoms of the disease. For influenza it is typically between one and four days, with two days being the average.

How long is flu contagious?

The period of time during which a person infected with the influenza virus can spread it to others begins one day before symptoms develop and can continue for up to 5 to 7 days after symptoms begin. Children may pass the virus for a longer period of time though.

Complications of flu (Flu complications)

Complications are the main reasons for the need for hospitalization and death resulting from flu. Additional infection and worsening of existing chronic disease with secondary effects are the most common types of complications that occur. The most serious infections are pneumonia and encephalitis. They can be a result of the influenza virus or a different pathogen. The different organism is usually a bacterium. Other common infections that sometimes occur involve the ears and sinuses. The worsening of chronic disease and secondary effects that commonly occur are the following:

The elderly (people greater than 65 years of age), residents of nursing homes, and children between the ages of five months and six years are at increased risk of developing complications.

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Flu diagnosis

A diagnosis can usually be made clinically, especially in the midst of flu season. There are other times though in which health care providers need to perform tests to confirm the diagnosis. Those situations are the following:

  • Uncertainty as to whether symptoms are due to influenza or another potentially serious disease
  • A person is at risk of a complication and is a candidate for antiviral therapy to reduce the risk
  • A health care provider plans to use an antiviral to reduce the severity or duration of influenza symptoms

Various laboratory methods are available but are costly and require time. In recent years, several different rapid tests have become available. They allow a doctor to make a diagnosis in the office in 15 minutes. They are not always as accurate and reliable as the laboratory tests. But overall they are useful to doctors for making treatment decisions. Also, they are considerably cheaper than laboratory tests.

Treatment for flu

Patients’ desire for antibiotics is a flagrant fallacy regarding treatment of the flu. Not only do antibiotics do nothing to kill or retard the growth of viruses. Their use in this setting has the potential of being harmful by promoting the emergence of resistant strains of bacteria which can cause a superinfection.

Antivirals on the other hand, do kill viruses and suppress their replication. The ones that are effective against the influenza virus work by blocking the actions of neuraminidase on the surface of viruses. Their main use is for the following:

  • Reducing the incidence of influenza complications in patients at high risk
  • Preventing or reducing the severity of symptoms in patients who did not receive the most current vaccine
  • Preventing influenza in patients who received the vaccine in years when the match between the vaccine and the circulating strains of virus is suboptimal

The flu vaccine and flu prevention

Antivirals notwithstanding, vaccination is the chief means of preventing the flu. Each year’s vaccine contains antigens of three strains of the influenza virus. Two strains are the most common circulating strains of the A virus at the time the vaccine is produced. The third strain is the most common strain of the B virus at the time of production.  The actual strains vary from year to year though, and may not be an exact match with the circulating strain at the time of outbreaks due to mutations.

Following vaccination the immune system of the recipient produces neutralizing antibodies. Those antibodies prevent the flu virus causing an outbreak from attaching to and entering respiratory tract cells of the recipient by blocking the HA protein of the virus if there is a reasonable match between the antigens in the vaccine and on the virus causing the outbreak. If the virus mutates significantly after the vaccine is produced though, the shape of its surface antigen will be changed to the point that the neutralizing antibodies won’t match up with it. Thus, they won’t prevent the new strain of viruses from entering and infecting the recipient’s host cells. In that case the vaccine won’t prevent infection. Another reason the vaccine might not prevent influenza is if the strain is the C type. The reason is the C virus is not used to produce vaccines.

A common fallacy of many vaccine recipients is the belief that the vaccine didn’t work because they “got sick anyway.”  In many of these instances though, the person did not experience the flu.  It was just a bad cold or other respiratory illness caused by a different virus.  Therefore they are not cases of vaccine failure. They are examples of the vaccine not performing beyond its intended purpose.














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