Usually, when viruses enter a living plant or animal cell, they grow, multiply and cause disease. Some can live in a cell without harming the victim (host) cell. But if the same viruses enter another kind of host, they produce disease. In addition to causing plant diseases, some viruses cause animal illness, as distemper in dogs, and colds, mumps and polio in man.
Viruses have various shapes. They may be spherical, rod-like or bottle-shaped. None of them can move by themselves.
Chemically, a virus consists of a core of NUCLEIC ACID, either RNA, DNA or both. This core is surrounded by a sheath of non-acid PROTEIN. Some viruses leave the sheath behind when they enter a cell. Others are believed to separate the sheath and acid core right after invading the host cell.
Once inside a living cell, viruses organize the cytoplasmic activities (enzyme systems) of the host and start to reproduce. Using raw materials supplied by the host cell, virus particles of RNA or DNA make copies of themselves.
Scientists have been unable to decide whether viruses are living or non-living. Because of their chemical composition, and their behavior within a cell, they appear to be living particles. It has been said that viruses are "cell nuclei in search of some cytoplasm." On the other hand, viruses can be crystallized. In this form, they are inert like non-living substances.
