Viruses - are fragments of DNA or RNA that depend on host cells that they infect for their reproduction. They are not cells. Viruses are thought to be parts of the genetic code that originated from either eukaryote or prokaryote cells. These code fragments contain enough genetic information for self-existence. At times, viruses are metabolically inert and technically non-living.
Viruses cause a variety of diseases in eukaryote organisms.
In humans they can cause smallpox, chicken pox, influenza, shingles, herpes, polio, ebola, AIDS, rabies, and some types of cancer. The four main types of biological entities described above share some unique characteristics that can allow us to distinguish them from non-living things. Organisms tend to be complex and highly organized. Chemicals found within their bodies are synthesized through metabolic processes into structures that have defined purposes. Cells and their various organelles are examples of such structures.