A flu virus appears to be an amazing thing as it is able to mutate quite quickly. There is some discussion whether the plague that Ukraine is experiencing now is H1N1, a mutation, or something entirely different. One factor they are investigating is total hemorragic destruction of the victims' lungs, in some cases, which is more typical of a bacteria of some kind rather than a virus, thus they are calling it a pneumonic plague. They are also seeing airborne transmission paths, even through the eyes.
From an article on the CDC's website:
Influenza A virus is capable of rapid genetic change in mammals. Its polymerase complex lacks proofreading capability, such that one in five virus particles produced is likely to contain a change at one of its approximately 13,500 nt (9). If such a change provides the virus with a competitive advantage, that strain quickly replaces its predecessor. In humans, the need to escape preexisting immunity exerts positive selection pressure on changes in amino acids comprising the antigenic sites of the surface glycoproteins, HA and neuraminidase (NA) (6,10). The process of progressive change in the antigenic properties of the virus is called antigenic drift and results in the emergence of an antigenically distinct variant strain every 2–3 years. Between drift epidemics, the influenza virus appears to be antigenically uniform (11), but the degree of genetic uniformity has not been studied extensively.This is pretty technical, but it is saying,
- 20% of flu viruses in mammals are likely to mutate,
- The mutation may quickly overtake the original,
- The presence of antibodies in humans encourages mutation,
- A new variant strain results every 2-3 years,
- The new variant is resistent to the antobodies of the original.