Invading the host cell enables the virus to

there a living thing. There alive but cant reproduce unlike most cells that are in your body. So they get in a host ... like your body, and are able to reproduce "multiply". If there were "dead" like the previous person stated then they would not efect us at all, hince why flu shots are actually "alive". Well the flumist is. Thats the live flu virus. Its just diluted (weakend) soo much it cant effect you at all. And if you ever come in contact with the virus itself in the outside world it wont hurt you becasue technically you already have it.