The make-up of a flu virus is somewhat complex. What researchers know about this virus is that it is roughly round, will the possibility of also being elongated or irregularly shaped. Inside the virus are eight segments of single-strand RNA that contain genetic code for making new copies of the virus. The flu virus has a layer of spikes that project from its surface, making it particularly striking. One of these spikes is the protein hemagglutinin (HA), which allows the virus to attach to a cell and start the infection process. The other is a protein called neutaminidase (NA) whose job is to enable newly formed viruses to exit the host cell.
These influenza viruses are classified as type A, B, or C based on their protein composition. You can find type A viruses in animals, as well as humans. The type B virus widely circulated among humans, and the type C has been found in humans, pigs, and dogs, and although it can cause mild respiratory infections, it does not spark epidemics.
The Type A influenza is the most dangerous and scary of the three types of viruses. It is responsible for the global pandemics of 1918, 1957, and 1968. Type A viruses are subcategorized into groups based on their two surface proteins, HA and NA. There are 16 HA subtypes and 9 NA subtypes.
The flu virus can be found in nature with several types of wild aquatic birds and has persisted in these animals for millions of years. The virus does not typically cause illness, however the flu viruses that frequently mutate can easily jump the species barrier from these wild birds to domestic ducks and then chickens. Pigs can then be infected with the avian influenza and the form that infects the human population. If humans and pigs live together in close proximity, then a mutation can occur when a pig is infected with the avian and human flu resulting in a virus whose genes have been resorted and can now spread from pigs to humans. Depending on the precise assortment of bird flu proteins that make it into the human population, the resulting flu may be more or less severe.
In 1997, it was discovered by scientists that the bird influenza skipped the step where it would have to infect a pig, and went directly to infecting humans. This alarmed health officials to think that a pandemic was possible. Fortunately at that time, the virus was not able to pass between people, and therefore did not cause an outbreak. Scientists are now speculating that chickens may be able to contract these human type viruses and cause the same problem.
The problem is that the influenza virus is constantly changing, be it small changes, or large and abrupt changes.
Type A and B influenza see continuous changes as the virus makes copies of itself. This antigenic drift is frequent enough to make the new virus unrecognizable to the human immune system, which is why a new flu vaccine must be produced each year to fight that yearâ€™s most common and persistent strains.
Type A influenza also frequently undergoes sudden changes, called antigenic shift. This occurs when two different flu strains infect the same cell and exchange their genetic material. The different assortments of HA or NA proteins in one of these shifted viruses can create a new influenza A subtype. Since the general population has little or no immunity to each new subtype, the appearance of such a new strain tends to coincide with very severe flu epidemics or pandemics. This is what makes the type A viruses so hard to prevent and vaccinate against. Frequently scientists donâ€™t know how the virus has mutated until an outbreak occurs.
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