A virus is a quantity of hereditary material enclosed in a protein shell. Viruses cannot reproduce independently of other organisms, rather only in living cells, therefore scientists do not agree on whether a virus can be considered a life form. The living cells in which a virus can reproduce can be anything from single-celled organisms to plants and animals. Not every cell can be used by every virus; the virus’ protein coat is used to recognize suitable host cells. Inside the host cell, the virus’ genetic material instructs it to make new viruses.

About one quarter of known viruses are capable of infecting plants. A plant can be infected by one or more viruses simultaneously. Several isolates of a virus can also be present in a plant at the same time.

A virus does not cause disease by consuming cells, or killing cells by poisoning, but by using cellular substances during multiplication, taking up space, and disrupting cellular processes.