Final answer:
Spike proteins on the surface of viruses facilitate the attachment to host cells by binding to receptor sites on the host cell membrane, allowing the virus to enter the cell.
Step-by-step explanation:
The structure that allows the virus to enter the host cell is typically a protein known as a spike protein. These spike proteins are part of the outer structure of the virus and are key in the process of attachment to host cells. They bind to receptor sites on the host cell membrane, which are molecules on the cell that the virus exploits for entry. Once a virus attaches to its host cell, it can penetrate the cell membrane, inject its viral genome, and begin replication. The influenza virus, for example, uses hemaglutinnin and neuraminidase spikes for attachment and release from host cells.