This Is a Certified Answer

Certified answers contain reliable, trustworthy information vouched for by a hand-picked team of experts. Brainly has millions of high quality answers, all of them carefully moderated by our most trusted community members, but certified answers are the finest of the finest.
On entering the host cells the viruses kills the head or important cell to take its place and they control the division of the cells and they force the cells to produce into the same cells[virus cells] thus harming the immune system of the person.
Because they act as non living organisms outside the host body and act as living organisms inside the host body