Diversity: Viruses
Viruses are often studied as organisms. However, if life includes the attributes order, reproduction, growth and development, use of energy, homeostasis, response to environment and evolutionary adaptation, then viruses are not living.
Viruses clearly have biological effects, e.g. disease, and for this reason
will be considered in this unit. We would argue that viruses have "shunted
off" many functions to their host enabling them to take advantage
of extremely small size. Alternatively, they may be plasmid-like organelles
that have become parasitic. 
to
size.
Structure
The simplest virus, sometimes called a viroid, consists of a strand of nucleic acid. The strand may be DNA or RNA and it may be single or double. If single, it will be found doubled around itself, thereby preventing degradation by enzymes in the cell. Viroids resemble plasmids. More complex viruses have a strand of nucleic acid surrounded by a protein shell. On entry to the host cell, the protein shell is lost. Some viruses have a membrane of host origin surrounding the protein shell. Viruses lack cytoplasm, or any mechanism to reproduce in the absence of a living host cell.
Importance
Viruses cause disease in many different organisms, from bacteria through to multicellular organisms. Viruses may also transfer nucleic acids from one cell to another.
