Sometimes there will be a protein core underneath the capsid which also surrounds the genetic material. Some viruses may have an additional covering on the outside called an envelope . An envelope is kind of like skin around the outside of the virus. If you examine a baseball, take it apart, you will see how some viruses are assembled. Yale University School of Medicine for a look at Adenovirus .
All viruses only exist and make more viruses.
And with the possible exception of bacterial viruses which kill harmful bacteria, all viruses appear to be harmful because their replication leads to the death of the cell which the virus entered. A virus enters a cell by first attaching to a specific structure on the cell's surface via a specific structure on the virus surface. Depending on the virus, either the entire virus enters the cell, or perhaps only the genetic material of the virus is injected into the cell. In either case however, the ultimate result of viral infection is the exposure of virus genetic material inside the entered cell. What things can become infected by a virus So far, there is not a living thing identified that doesn't have some sort of susceptibility to a particular virus.