Answer & explanation:
The role of viruses is to infect specific cells and alter their metabolism in order to produce more viruses, from which, at a certain moment, the cell "bursts". Once this happens, thousands of new viruses repeat the process in other healthy cells.
The increase in the amount of virus in a organism (viral load) consequently increases its ability to cause symptoms (virulence), at a certain point, when the viral load reaches a specific amount, the body is able to produce defenses on its own (in most cases, virus treatment occurs this way).