This week I bring you a topic that is always in fashion: the flu virus. As you will have heard on TV some time ago, in most of Spain, the epidemic level of this disease is occasionally reached. Today I want you to understand perfectly what is, without a doubt, one of the most common viruses in our society…
First of all, what does epidemic mean?

An epidemic is a sudden and temporary increase in the frequency of a disease above the expected level in a geographically limited population.

And a pandemic? Does this word sound familiar to you?
It is also an increase in the frequency of a disease, with the difference that it applies to a large population in a geographically large region. An example? The pandemic caused by the influenza A virus (also called then avian influenza) that took place between 2009 and 2010. This virus traveled the world for 14 months, leaving 19,000 fatalities along the way.

But what is a virus?
Those of you who frequent my blog will know exactly what it is about, but for the newer ones: Viruses are very, very small agents that need to infect a cell to multiply. One thing must be very clear, and that is that a virus is not a living being like a bacterium, an algae or a dog. Why? Because it is not capable of performing the main vital functions: nutrition, relationship and reproduction.

And what is a virus made of? Of a nucleic acid (DNA or RNA) in which all the genetic information of the virus is collected and an envelope that protects it. In addition, depending on the type of virus, other additional and less important components can be added.

And now yes, the flu, that disease that we all know and that is a respiratory infection with moderate fever, general malaise, myalgia, pharyngitis and non-productive cough, is caused by viruses corresponding to the Ortomixoviridae family (families of viruses end with –viridae).

Within this family we have 5 genera, 3 of them, called Influenza A, Influenza B and Influenza C, are the causes, respectively, of influenza A, B and C. Despite the fact that we call the 3 diseases the same, only A is the real flu, and the one we’ll focus on from now on.

The influenza virus is characterized by having in its envelope two very important proteins with very rare names: hemagglutinin and neuraminidase. Hemagglutinin has the ability to bind to a protein on the surface of cells that it will infect. Neuraminidase, in turn, has the ability to break down sialic acid. And you will wonder, what does this acid have to do with all this fuss?

The mucus that covers the entire external surface of our respiratory system is rich in sialoproteins, that is, proteins conjugated with sialic acid. The mucus is, precisely, a defense mechanism of the organism since it agglutinates the viruses preventing its dissemination. The mission of the virus’s neuraminidase is to break this mucus to allow it to spread through as many cells as possible.

And now that we know this we can understand why we should be vaccinated every season of the flu virus. The reason is quite simple: these two virus proteins change from season to season by different mechanisms. The consequence? The virus is totally different. To give you an idea: from the first protein there are, for the moment, up to 15 variants, and from the second 9. The result? The existence of a lot of types of influenza viruses.

When you get vaccinated for this disease, you do it for one of these specific types of flu viruses. In this way, the body responds by generating specific antibodies against this virus. If the same virus infects you, your immune system will be prepared and will respond by killing it.

The problem of the influenza virus is this great variability that it presents. If the type varies from season to season, the antibodies you have generated will not be able to recognize the new virus and therefore, you will be able to get it again despite being vaccinated.

In fact, the influenza virus is assigned a specific name depending on the type of neuraminidase and hemagglutinin it has. For example, if you have type 3 hemagglutinin and type 1 neuraminidase it will be called the H3N1 influenza virus.

And to put the icing on the cake, epidemics and pandemics, which I told you about at the beginning, are associated, precisely, with the change of these proteins on the surface of the virus. A change in the virus will mean that a large part of the population does not have antibodies against it despite being vaccinated and, therefore, is not capable of generating an effective immune response.

While the slightest changes in these proteins cause epidemics, the most drastic changes cause pandemics. It is easy to understand: the rarer the change, the less likely it is that there will be people immune to it and therefore the greater the contagion.

As curiosity and to finish? The reservoir, that is, the place where this virus accumulates naturally, is birds. Through these animals the virus can spread to pigs. And it will be precisely in them that the changes in the famous two proteins will take place. From pigs, the virus will spread to humans. This explains, precisely, why the areas in which new influenza pandemics arise are areas in which the population maintains close contact with pigs.