Viruses can only replicate by using the host cell apparatus as they are non-living.A virus consists of DNA or RNA enclosed in a protein coat or capsid.Viruses attach to the host cell using attachment proteins or glycoproteins on their surface and inject their RNA or DNA into the host cell.This is usually accompanied by a few viral enzymes which help the virus to replicate.RNA viruses have RNA that can be used to make viral proteins and copied several times to make new viruses.DNA viruses must have their DNA copied to mRNA before protein synthesis occurs.