Influenza virus

Influenza, the virus that causes the flu, is one of the most common infectious diseases.

Now, there are three types of influenza that infect humans, called type A, type B, and type C, each one with slightly different genome and proteins.

Influenza belongs to the virus family Orthomyxoviridae - and type A and B have genomes that are made up of eight RNA segments, whereas type C, has a seven-segment RNA genome, with each segment containing a few genes.

Now, type A, the most common type of influenza virus, can be further subdivided based on two of the glycoproteins on its protective envelope surface; H protein, or Hemagglutinin, and N protein, or neuraminidase.

Hemagglutinin and Neuraminidase can vary a bit in their structure, so different versions are identified by a number.

For example, type A subtype H3N2, sometimes just called H3N2, has hemagglutinin number 3 and neuraminidase number 2 on its surface.

H3N2 and H1N1 are the most common type A subtypes to infect humans, but they both also infect various animals.

To give the full name of a virus, we use the type, the original host that it came from, the location where the virus was first identified, which is usually a city, the strain number, the year of origin, and—for type A influenza—the subtype named by the H and N glycoproteins.

For example, an H1N1 type A flu virus of duck origin from the province of Alberta, Canada, that is the 35th strain discovered in 1976 would be called A/duck/Alberta/35/76 (H1N1).

Type B influenza is less common, it only infects humans and doesn't mutate as often as type A.

Type B influenza only has a few types of H and N glycoproteins on its surface.

Therefore the naming pattern is similar to type A influenza without the H and N subtype included at the end or the host type, since it only infects humans.

For example, a type B virus found in Yamagata, Japan, which is the 16th strain discovered in 1988 would be called B/Yamagata/16/88.

Finally, there's type C influenza which is only one species, and is the least common and least likely to mutate of the three.

Influenza C usually causes mild disease in children, and unlike type B, it can affect both humans and pigs.

Rather than hemagglutinin and neuraminidase, type C influenza uses a hemagglutinin-esterase-fusion protein to enter and exit cells.

So, type C influenza is named without the HN subtype, similar to how type B's written.

For example, a type C virus found in Sao Paulo, Brazil which is the 37th strain discovered in 1982 would be called C/Sao Paulo/37/82.

Of the three types, type A is the most common and causes the most severe illness.

One reason is that the virus has a tendency to mutate its H and N glycoproteins during replication, and this allows daughter viruses to form that are slightly different from one another and from the parent virus.