Bird Flu Virus.
 
Bird flu or avian influenza, as it is known to medicine, is caused by an influenza virus. It mostly affects domesticated birds and occasionally wild birds and domesticated animals and pets, it rarely infects humans.

But it is the possibility of an avian influenza pandemic that makes things serious and frightens people into panic.
 
So when the situation is bad anyway, why do we need to know about the viruses anyway?” you might be asking. Well, not all flu viruses are lethal and very fewer of them actually circulate among humans.

Most of them confine themselves to birds and animals. Wild birds naturally harbour the influenza viruses but very rarely get sick from them.
 
But the threat of this changing goes from bad to worse and a pandemic outbreak is looming large, so it's important that people are aware of what bird flu is, and how it affects the human population.

To begin with, there are many types of influenza viruses, sub-types and strains.
 
Influenza viruses are of three types A, B and C. Of them, the type A viruses are divided into subtypes based on the two surface proteins of the virus: hemagluttinin (HA) and neuraminidase (NA).
 
There are 16 known hemagluttinin subtypes and 9 known neuraminidase subtypes of surface proteins. Different combinations of HA and NA proteins are responsible for different subtypes of influenza A virus.

For example, the H7N3 virus is formed with the 7th hemagluttinin (HA 7) protein and 3rd neuraminidase (NA 3) protein.

Similarly, the much dreaded currently endemic virus H5N1 is made with HA 5 and NA 1 proteins.
 
Only H1N1, H1N2, and H3N2 subtypes of influenza A virus are in circulation at present among humans. While other subtypes are mostly found in animals. For example, H7N7 and H3N8 viruses infect horses whereas H3N8 found to be infectious to dogs.

The following are some important influenza A subtypes.

H5 subtype: Of the nine known subtypes of H5 viruses, H5N1 virus is most lethal and is behind the current epidemic in Asia and Europe. It have been documented since 1997 and found to have mutated considerably in that time.

H7 subtype: The nine potential known subtypes of H7 rarely infect humans but can occur among people who have direct contact with infected birds. These may either be low pathogenic or highly pathogenic with symptoms of conjunctivitis and upper respiratory tract ailments.

H9 subtype: The nine known subtypes of low pathogenic H9 viruses rarely infect humans.

Influenza (Type): Influenza B viruses are not classified into subtypes. Unlike influenza A viruses, which are mainly bird viruses, these Influenza B viruses are found only in humans.

These are usually much less severe than their A counterparts but can cause morbidity and mortality among humans. Certainly, they can’t whip up pandemics as can their more serious counterparts.

Strains of Influenza Viruses:

Subtypes of influenza A virus and Influenza B virus are further classified into many different strains. New strains of the influenza virus replace older ones as they form in a molecular genetic process called antigenic drift.

Knowing the strain of a virus is important because antibody protection will develop only after infection, and antibodies from an older strain of the virus may not provide protection against a new strain.

Similarly vaccines developed for one strain may become ineffective against new strains. This is why it is so difficult to produce a cure for the flu generally. It mutates so easily and so quickly, that vaccines very quickly become ineffective.

Influenza Type C: Like Influenza B viruses, the type C viruses are also not classified into subtypes. They cause mild illness in humans and do not cause epidemics or pandemics.