Is a virus alive or not?

I have sometimes wondered whether a virus counts as being alive or not.

I know that when they are inside us like a cold infection or similar they act as though they are alive, and clearly can have a big and devastating effect and even kill people.

But then the fact that they need something else to function strikes me as odd, as does the fact that some of them can apparently just stay around as it were doing nothing for millions of years then suddenly reactivate.

So are they alive or not? Would be interesting to hear what people think!