The flu is caused by 3 types of RNA viruses: influenza types A, C, and B. This disease is often called the flu and is generally caused by two viruses: A and B. They are transmitted by close contact with infected animals or via aerosols from infected individuals. Subtypes of influenza B and A viruses can be further divided into strains. There is a wide range of influenza A subtypes as well as influenza B virus strains. New strains of influenza virus can appear and replace the older versions. Such new viruses have antigenic variations referred to as antigenic shift and drift.
Viruses under influenza A are known to infect pigs, people, horses, seals, whales, birds, and many other animals. Wild birds usually represent the natural hosts for influenza A viruses. The virions of this virus comprise of 3 membrane proteins: NA, M2, and HA. Moreover, they also contain a ribonucleoprotein core – with eight viral RNA segments and 3 proteins (PB2, PA, and PB1); a matrix protein that is underneath the lipid bilayer; and the NEP protein. Influenza A viruses can be split into subtypes based on 2 membrane proteins on the virus’ surface: NA and HA. There are 11 different NA subtypes (NA1 to NA11) and 18 different HA subtypes (HA1 to HA18).
Although many different combinations of NA and HA proteins are possible, only a few influenza A subtypes are in circulation among people – H1N1 and H1N2. More subtype combinations can be found in different animal species. For instance, H3N8 shows illness in seals and dogs and H7N7 elicits the disease in horses. Subtypes of this virus are named by combining their NA and HA surface proteins. H3N8 has NA8 and HA3 surface proteins while H5N1 has NA 1 and HA 5 proteins.
These viruses are responsible a similar spectrum of disease as influenza A. Influenza B viruses, however, do not result in pandemics. This is because the virus has a limited host range – humans and seals; this limits the emergence of new strains through re-assortment. Moreover, influenza B viruses are not divided into subtypes; they are broken down into strains and lineages. The influenza B viruses that are currently in circulation include B/Yamagata and B/Victoria. The virions of influenza B contain four proteins: NB, BM2, NA, and HA. The BM2 protein is necessary for the process of uncoating. The NB protein, on the other hand, is an ion channel. Therefore, it is not required for cell culture viral replication.
Influenza B is responsible for significant illnesses. In 2008, approximately 1/3 of lab-confirmed cases of the flu were caused by influenza B. for this reason, the seasonal flu shot contains a component of influenza B.
The enveloped virions of influenza C virus have hexagonal structures. Additionally, they create stretched cordlike structures when they sprout from the cell. What is this virus comprised of? It has four proteins and viral RNA. M1 protein is just beneath the membrane and CM2, which is an ion channel. Influenza C does not have separate NA and HA proteins but their function is merged in a protein called HEF. This means that its virion has 7 RNA segments instead of 8.
Influenza C infections usually result in a mild respiratory illness, which can be compared to other respiratory viruses. This virus does not result in epidemics. According to certain studies, almost all adults have been infected with this virus. Vaccines against this influenza type are not available and lower respiratory tract complications are uncommon.