Viruses are like the fast engineers that can cleverly take over whole machinery and use it for their purpose. Clever able to mutate bad virus gets into the cell and replicates making the cell die. The whole specs of virus is that it needs a cell to replicate.
There are milions of viruses all over the world. Varing in kinds they have one thing in common. As they exist outside the cell the they are called as virons are build as core RNA or DNA molecules , a proten coat called capsid, that surrounds and protects genetic material. Sometimes additionelly the protein is also covered by such called envelope of lipids.