The major reason that many biologists do not consider viruses to be alive is the fact that they can not reproduce on their own.
Instead of reproducing on their own (either sexually or asexually), viruses have to use their hosts' cellular mechanisms to reproduce themselves.
This process of reproduction is how viruses cause their hosts to become sick. Viruses take over the cells and force the hosts to produce more and more copies of the virus. Depending on the characteristics of the virus, this can do anything from making the host very sick to having no effect at all.
Virus is an organism that lives in a cell of another living things. Virus are so small and have so simple structure that many scientists do not consider them to be both living and non-living things. Primary reason for considering virus as non living things because they are incapable of any activities associated with living organisms without depending on cell of some other life form.
A virus is incapable of living having any activities associated with life when it exists by itself without support of other life form. In that condition is is just like a lifeless particle. However inside another living cell it becomes alive and reproduces very fast.