what is a virus? why are they not considered to be truly alive?

they aren't living because they need a host to reproduce and can not successfully "live" with out a host. living things... grow, reproduce, need energy

A virus is basically just a strand of RNA (a ribonucleic acid similar to DNA) with a protective shell and an injecting mechanism. When it finds a host cell, it latches on to it and injects its genetic sequence into the cell. The virus' RNA then fuses with the cells DNA, which gears the cell to producing more viruses. Eventually, the viruses break out of the host cell and start the process over. It is not considered alive because it does not have all of the characteristics of living things

some characteristics of living things:

1. Grows and develops-(virus does not develop. The host produces viruses)
2. Made of cells (viruses are not made of cells)
3. Able to support itself  by using energy
4. Able to pass on its genetic code through reproduction (virus can do this, but only with the help of a host)
5. Able to respond to its outside environment