An obligate parasite is an organism that cannot live independently of its host. For example, a virus is an obligate parasite because it cannot reproduce outside of a host cell. In biology and ecology, an organism (in Greek organon = instrument) is a complex adaptive system of organs that influence each other in such a way that they function as a more or less stable whole and have properties of life. ... The human immunodeficiency virus (HIV) A bacteriophage virus A virus is a submicroscopic parasitic particle that infects cells in biological organisms. ... Cells in culture, stained for keratin (red) and DNA (green). ...
The study of viruses is known as virology, and those who study viruses are called virologists.
Viruses are similar to obligate intracellular parasites as they lack the means for self-reproduction outside a host cell, but unlike parasites, which are living organisms, viruses are not truly alive.
Over time, genes not required by their parasitic lifestyle would have been lost in a streamlining process known as retrograde- or reverse-evolution.
Viruses are obligate intracellular parasites meaning that they can only reproduce by invading and taking over other cells as they lack the cellular machinery for self reproduction.
The term virus usually refers to those particles which infect eukaryotes (multi-celled organisms and many single-celled organisms), whilst the term bacteriophage or phage is used to describe those infecting prokaryotes (bacteria and bacteria-like organisms).
Whether or not they are "alive", they are obligateparasites, and have no form which can reproduce independent of their host.
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