Re: Are viruses living things? If they are, what is their life span?

Dear Nancy,

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 memebrane 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).

Hope this answers your question,
Rob Welton