From: Zardoz ®	14/11/2001 17:22:53
Subject: re: MEDICAL FAQ NOTES	post id: 496179
Are viruses living things? Viruses just "sponge off" their host cell for these functions, and more. This is a question that scientists have been asking for many years. I'll start by giving the general criteria used for describing a "living" organism (Invitation to Life,: H. Curtis and N. Barnes, Worth Publishers, 1994) 1/ They have the capacity to replicate themselves. 2/ They have enzymes, and complex proteins essential for the chemical reactions needed for life. 3/ Have a membrane that separates the cell from the environment and gives it a distinct chemical identity. Usually for an organism to be considered living it must be able to carry out all the three criteria on its own. With regards to viruses, they are unable to replicate themselves without the aid of a host cell (the cell they invade). Out side of the cell they are just inert particles. The only way in which they can replicate is to invade a cell, take over its replication machinery, and use it to produce more virus. In essence most viruses are just nucleic acid (RNA or DNA) surrounded by protein. Some viruses to have their own enzymes but the enzyme itself is not active until the virus enters its host cell. Outside of the cell, they usually do not have a metabolism of their own. With this in mind, scientists do not consider viruses to be living. They are thought of as inert particles and only function and replicate with the aid of a host cell. As far as life span, since they are living, they don't have a life span, as do bacteria, mammalian or plant cells. However, if a virus has access to a host that it can infect, theoretically it can keep replicating itself until it no longer has a host (either by killing its host or decreasing the numbers of hosts). Rob Welton, Grad student, Molecular Biology and Genetics, Boston College http://www.madsci.org/posts/archives/aug98/898786228.Bc.r.html Are viruses living things?