Final answer:
A virus attaches to a specific receptor site on the host-cell membrane through attachment proteins in the capsid or via glycoproteins embedded in the viral envelope.
Step-by-step explanation:
A virus attaches to a specific receptor site on the host-cell membrane through attachment proteins in the capsid or via glycoproteins embedded in the viral envelope.
The specificity of this interaction determines the host—and the cells within the host—that can be infected by a particular virus. This can be illustrated by thinking of several keys and several locks, where each key will fit only one specific lock.