Virus are made with ADN or RNA only, which both of them has the information of replication. Once that the virus is in a cell, this ADN or RNA will be deposit inside the cell at the replication system that is going to make the virus reproduce unlimited times until the membranes of the cell break down. And they are useful because you can use them as a carrier or vector for treat diseases or for transgenics, bioterrorism, and so on.