What is the function and structure of a virus?

A virus is a parasite that can only live inside another cell. It takes over the host cell's machinery to transcribe and translate the proteins it needs to produce new viruses. Each virus can only infect one specific type of cell because it gains entrance into a cell by binding to specific receptors on the cell surface.

A virus consists of DNA or RNA enclosed in a protein coat called a capsid. Some viruses also have a viral envelope that is derived from membranes of host cells, cloaks the capsid, and aids the virus in infecting the host.