Explain why many biologists do not consider viruses to be living organisms.

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.