Why does a virus need a host cell in order to replicate?

Answer 1

We don't know. We can't really study how they evolved because they don't leave fossils. There are several different theories as to how they evolved - they could have been bits of cells, or random self-reproducing molecules, or particular cells that lost all of their useful parts except for DNA/RNA.

answer 2:

Probably viruses exists longer as life (a we define it today) itself. Probably the first mechanisms of life are early forms of DNA/RNA without a skin. I think the earliest forms of life started in an area with a source of the same building blocks and an area dense but liquid enough to carry them like water. To me it sounds logical that futher away from this source, where building blocks became scarce, these molecules could not reproduce anymore. By time, chance and exemplary differences, only the ones that adapt to the changed environment could carry themselfs futher away, benefit from that and adapt (develop) even more. The ones that developed a skin which acted like a sealed buffer with building blocks as a reserve within and became more self supporting, could carry itself to other places and develop even more from that. This cellular form of life is related to the life as we know it today and is also more developed and abundant. Probably the skinless forms didn't go away completely. They invaded those buffers and made use of it as well and now they still exists as viruses today.

Interesting to know is that sea is full of viruses (1 to 10 billion per 1 liter of water) and play a large role in that ecosystem.

Answer 2

Just like people need to reproduce, it is inherent in nature for other types of organisms to reproduce as well. The ultimate question (i.e., "Why is there life?") is yet to be answered by science. Many philosophies and religions attempt to answer this, sometimes in very different ways. It is part of the exciting mystery of life.

Answer 3

It takes over the protein production mechanisms & nucleic acid duplication mechanisms of the cell it infects to reproduce. Without those cellular mechanisms the virus is just an inert particle of nucleic acid and protein that does nothing.

Answer 4

There is no other living thing that requires a virus in order to reproduce.

Answer 5

A virus needs a host cell because it can not produce more of itself without the host. It is as if you could not be able to think if you had no brain, or walk without legs.