Viruses do not have all of the characteristics of living things.
They consist of a protein coat which contains either DNA or RNA.
They are not made of cells. They have no cellular structures. They
do not require nutrients. They do not have metabolism. They do not
grow or develop. They do not reproduce on their own. They must
high-jack a living cell, inject it's genetic material, which then
takes over the host cell which then becomes a virus factory.
Eventually the cell becomes so full of replicated viruses that they
burst, releasing the viruses so they can go on to attack other
A virus is not considered a living thing because it can't live on its own and has to rely on another animals cells to reproduce. A living organism technically has to be able to survive and reproduce on their own without help from other organisms. They don't use their own energy to grow, make food, take food, or produce waste.
Viruses are not considered living organisms because they are not composed of cells (the Cellular Theory of Life). Also, viruses cannot replicate independently - they must infect a living cell before their structure and genetic material can be reproduced and multiplied.
However, there is a vigorous ongoing debate about whether or not
viruses are living - they do have discrete genetic material that is
transmitted from "parent" to "offspring", they can react to the
environment by changing what proteins they are expressing and some
viruses are extremely complex.
One of the main reasons viruses are considered non living is because they cannot replicate by themselves. In order to replicate they must find a host cell and inject DNA into the host cell which disrupts the host cell's normal processes by causing it to make copies of the virus instead of carrying out all its normal processes. Once the viruses have been created inside the host cell, the host cell bursts to release the viruses. This is the same reason it is considered parasitic, since it uses the host cell for its benefit and damages or kills the host cell in the process.