Final answer:
The structure found only in viruses that infect animal cells is the viral envelope. It is derived from the host's membrane structures and contains glycoproteins for attachment to the host cell.
Step-by-step explanation:
The structure unique to viruses that infect animal cells is the viral envelope. This envelope is derived from the host's membrane structures and may be of intracellular or cytoplasmic origin. The capsid and nucleic acids are found in all viruses, but the viral envelope is specific to certain types of viruses, often those infecting animal cells, and is responsible for aiding in the entry of the virus into the host cell. The envelope typically contains glycoproteins that recognize and bind to receptors on the surface of the host's cells.