Is a virus a poison?
Although &quot;Virus&quot; literally means &quot;Poison&quot; in Latin, it would be a great over-simplification to regard a Virus as a Poison.
Now, most poisons are chemical compounds that interfere in some chemical pathway in our body. For example:

Carbon Monoxide: Binds with Haemoglobin, rendering it unable to transport oxygen
Sarin: Indirectly inhibits degradation of the  neurotransmitter &quot;Acetylcholine&quot;
at the synaptic cleft.   Mechanism of action of Sarin

Hence most poisons are quite &quot;simple&quot; in their actions. However, viruses can be considered as a complex machinery of functions that interfere with a multitude of processes in our body including cell transcription, receptor-mediated endocytosis, et.c
Hence, the best analogy would be to consider a virus as a tactical poison.
Now, as for last question, viruses are fragile assembled structures.  Viral Self-Assembly

Environmental factors like heat, uv-light, oxygen, etc may damage the assembled structure. Hence, viruses have a self life, and so do may poisons.
  Sarin (self life)
However, poisons tend to have a longer self life because chemical compounds are more stable than the capsid of viral proteins with weak hydrogen bonds.
As per David 's suggestion, poison is defined as &quot;...substances that cause death, injury or harm to organs, usually by chemical reactions or other activity on the molecular scales, when an organism is exposed to a sufficient quantity.&quot;  Wikipedia
Viruses do work at molecular level, but not in the same sense as poisons do. Hence, viruses do fit into the description of poisons but not precisely.
Hope you find this helpful.