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Question:

Why does the virus need a host in order to multiply?

Use the cells' resources

Explanation:

A virus is typically a is a DNA or RNA moiety encased in a protein capsule. The capsule is usually specific to the class and type of virus and it lends to the virulence of the entity. In order for infection to be established the virus must bypass the host's immune system and it's immune response and invade a host cell. There it will incorporate it's genetic material into the hosts' nucleus and harness the machinery that it does not in itself possess in order to make multiple copies of itself. It uses the host cell's DNA replication machinery, proteins amino acids and energy among other things before it bursts out of the cell and destroys it. This continues for some time and eventually the host may perish. The virus will then be passed along to a different host where it's replication using that hosts machinery will continue.