How does a virus harm a cell?
There are different types of virus but they all want to do the same thing; multiply. In entering the cell a virus can destroy the cell walls or coating to inject or attach itself. In multiplying within the cell they can rupture it entirely. In infecting a call a virus can stop it from functioning normally. In the cases of retroviruses, they alter the genetic function of cells to cause it to make more viruses and this can cause the cells creation of normal parts to malfunction, possibly even causing the cell to become cancerous.