Virus is a kind of acellular microorganism, which is tiny and has no complete cell structure. It contains a single type of nucleic acid (DNA or RNA) and must be parasitic and replicated in living cells.

Like all living things, viruses have inheritance, variation and evolution. It is a life form with a very small volume and a very simple structure. Viruses are highly parasitic and depend entirely on the energy and metabolic system of host cells to obtain the substances and energy needed for life activities. When it leaves the host cell, it is just a large chemical molecule. When it stops moving, it can be made into protein crystals. It is an inanimate object. When it meets the host cell, it will enter, copy and assemble through adsorption.

The life activity of virus is very special, and it absolutely depends on cells. It exists in two forms: one is extracellular and the other is intracellular. When it exists in the extracellular environment, it does not show replication activity, but maintains infection activity, and exists in the form of virus bodies or virus particles. When it enters the cell, it releases nucleic acid molecules (DNA or RNA) and replicates in the form of nucleic acid molecules in a unique life activity system under the condition of intracellular environment.