What is the flu virus? Definition

Influenza or the flu virus can be defined as a virus that causes the flu disease, being extremely contagious because the virus spreads very easily from animals to man and from one person to another. It primarily attacks the upper respiratory tract and the lungs.

The flu virus, which is an RNA (Ribonucleic acid) virus is easily transmitted from person to person through air or by direct contact, or through surfaces, which have been contaminated by the infected person.

The flu virus structure

The structure of the flu virus is somewhat variable.

It is spherical or ovoid in shape and covered by a lipid envelope, which is drawn from the plasma membrane of the host cell and in which it multiplies.

The lipid membrane has spikes made of glycoproteins proteins. They are called haemagglutinin and neuraminidase, which determine the serotype or the subtype of the virus, an example being the H1N1 of the flu virus A.

About 80% of the spikes are haemagglutinin, which helps the virus to attach itself to the surfaces of the host cell.

In mammals, it is the epithelial cells in the nose, the throat, and the lungs. In birds, it is the cells of the intestines.

The receptor of the host cell is sialic acid – a small sugar that is attached to various different proteins on the cell surface

The rest of the 20% or so spikes are neuraminidase, which facilitates the release of the newly formed virus particles from the host cell.

Beneath the lipid membrane is a matrix made up of mostly proteins, which give the lipid membrane its strength and rigidity. This is called M1 or matrix protein.

In the case of the influenza virus, seven or eight strands of RNA are tightly encapsulated, waiting to be released into the host cell. The RNA is bound onto coils of ribonuclear proteins (RNPs).

Size: The virus measures 80 to 120 nanometers in diameter.

Types of flu viruses and the potency of each type

The flu virus is classified into three types:

  • Influenza virus A
  • Influenza virus B
  • Influenza virus C

A and B types of flu viruses have an unstable genetic structure and therefore, mutate and change into new strains making it difficult to control them.

The influenza A virus is the one that mutates the most. Due to this property of these viruses, it becomes difficult to treat them with a drug, which was used on them before, because with mutation, their properties also change and the drug does not work.

The virus, therefore, is able to invade our body because our body has not developed immunity against the newly mutated virus. You are, therefore, susceptible to catching the flu infection throughout life.

Similar is the case with the vaccines. The vaccine used in this year will not be effective in the following year because the virus has mutated and a new vaccine has to be made for it to be effective against the new strain.

Most of the flu viruses originate and mutate in Asia and Africa, where closeness between humans and livestock is more proximate. This makes it convenient for the viruses to spread and mutate between humans and animals.

Together these viruses are antigenically distinct from one another but form their own viral family, Orthomyxoviridae.

Influenza A virus

It is the “A” type flu virus, which is responsible for most of the sickness and flu epidemics in humans with the B virus also contributing in a small way.

Influenza A viruses are divided into subtypes or serotypes (distinct variations) depending on the type of the two proteins present on the surface of the virus and the response of the antibodies to the virus invasion:

  • The hemagglutinin (H) and
  • The neuraminidase (N).

There are 18 different hemagglutinin subtypes and 11 different neuraminidase subtypes.

You, therefore, have H1 through H18 and N1 through N11.


  • H1N1
  • H2N2
  • H3N2
  • H5N1
  • And more

The influenza A virus undergoes frequent mutation and, therefore, the old flu vaccinations do not work. New flu vaccinations and newer drugs need to be developed every year and people need to take these vaccinations every year.

Of these, H1N1, H1N2, and H3N2 are the only known strains to circulate among humans.

It was the H1N1 serotype of the “A” flu virus, which caused the swine flu pandemic of 2009. This pandemic had its origin in Mexico and was declared over by WHO in 2010.

Bird flu, human flu, swine flu, equine flu, and canine flu are some of the names given to the sickness caused by the flu virus A variants.

The Center for Disease Control and Prevention offers a useful article on how the flu virus can change and mutate.  The A virus also infects other animal species, like birds, and is known to cause severe epidemics in poultry.

Influenza B virus

The “B” virus is not as dangerous or common as the “A” variety. It mutates much more slowly than the “A” variety and has only one serotype.

As a result, you develop immunity against this type of virus quite early in life. It is seen to infect only humans (and seals), unlike the A virus which is seen to infect animals also. The influenza B virus causes only localized outbreaks in crowded places like nursing homes.

Influenza C virus

This is the least common virus seen among influenza viruses. Its infective virility is quite low and is seen to cause mild respiratory infections in children only. However, it is seen to cause epidemics in dogs and pigs.

The influenza C virus is morphologically and genetically different from the other two viruses.

According to the CDC, the seasonal flu vaccine does not offer protection against the influenza C viruses.

How do the flu viruses mutate?

There are two ways that bring about changes or mutations in the flu virus

Antigenic drift

Antigenic drift is a type of variation in the genes of the viruses, occurring cumulative mutations in the virus genes.

This is the change that occurs every season of the year. It occurs slowly over time and is responsible for seasonal changes that occur in the virus every year.

Antigenic shift

There is another type of change, the flu virus undergoes. It is called the antigenic shift.  This occurs occasionally, but suddenly and when it does occur, it can cause a worldwide epidemic called the pandemic. This is because there is no protection for this type of changed virus throughout the population.

This change occurs when two different flu strains of the virus combine and infect the same cell. This is the type of mutation, which allows flu viruses to spread from animals to humans.

How long does the flu virus live in your body?

Once you are infected, the flu virus is usually alive in your body for a few days or weeks in normal healthy individuals — that is till the flu disease lasts. Our immune system takes care of that.

However, certain other viruses are known to lie dormant or asleep in your body for years or even a lifetime without causing any symptoms or complications. They do not multiply nor cause any infection. But, not so with the flu virus. It is completely destroyed by the body’s immune system.

How long does the flu virus live on surfaces of objects?

The influenza A virus can stay alive on the surfaces of objects. However, the time period varies on the type of surface the virus has been deposited

  • On hard surfaces such as hard plastic or metal, the virus can live for one to two days. Examples include doorknobs and telephone instruments.
  • On porous materials such as cloth (handkerchiefs) or paper, it can survive for less than 8 to 12 hours
  • On paper tissues for 15 minutes
  • On skin for about five minutes
  • However, moist surfaces can keep the virus alive for up to 72 hours.

History of influenza virus epidemics and pandemics

The world has seen pandemics during the last century, which took a large number of lives. Here is the list of the epidemics/pandemics and the figures for deaths that occurred in the United States and worldwide.

  1. 1889 Russian Flu Pandemic – about 1 million flu deaths
  2. “Spanish flu” A  of 1918-19 caused the highest number of influenza-related deaths: approximately 500,000 deaths occurred in the U.S. and 20 million worldwide
  3. “Asian flu” A of 1957-58 caused 70,000 deaths in the United States and about one million to two million deaths worldwide
  4. “Hong-Kong flu” A of 1968-69 resulted in 34,000 deaths in the United States and an estimated one million to three million people died worldwide.
  5. 2009 H1N1 Flu Pandemic – about 18,300 deaths in the United States and up to 203,000 deaths worldwide.