A virus is a small infectious agent that can replicate only inside the living cells of organisms. Most viruses are too small to be seen directly with a light microscope. Viruses infect all types of organisms, from animals and plants to bacteria and archaea. Since Dmitri Ivanovsky's 1892 article describing a non-bacterial pathogen infecting tobacco plants, and the discovery of the tobacco mosaic virus by Martinus Beijerinck in 1898, about 5,000 viruses have been described in detail, although there are millions of different types.Viruses are found in almost every ecosystem on Earth and are the most abundant type of biological entity.The study of viruses is known as virology, a sub-speciality of microbiology.
Virus particles (known as virions) consist of two or three parts: the genetic material made from either DNA or RNA, long molecules that carry genetic information; a protein coat that protects these genes; and in some cases an envelope of lipids that surrounds the protein coat when they are outside a cell. The shapes of viruses range from simple helical and icosahedral forms to more complex structures. The average virus is about one one-hundredth the size of the average bacterium.
I'm probably wrong but sometimes when a cell or virus reproduce, sometimes there are mutants (the ones that came out the wrong way or are missing body parts) and eventually the mutant become species of their own. Like evolution.
Viruses are clever, they can mutate and adapt to any environment. As soon as scientists figure out a way to combat the virus, the virus can then mutate to counteract the cure.
This is not the case with all virus strains.
they evolve-natural selection