There are two mechanisms by which enveloped viruses enter host cells. In one of the mechanisms, the virion attaches to host cell receptors by specific proteins on its surface, called spikes. The envelope of the virus fuses with the plasma membrane of the host, and the nucleocapsid is released directly into the cytoplasm. The nucleic acid then separates from the protein coat.
In the second mechanism, the enveloped virus adsorbs to the host cell by specific proteins on its surface, and the virion is taken in by endocytosis. In this process, the host cell plasma membrane surrounds the whole virion and forms a vesicle. The envelope of the virion then fuses with the plasma membrane of the vesicle, and the nucleocapsid is released into the host cytoplasm.
The capsid protein is then removed. releasing the nucleic acid of the virus. A naked virion also enters by endocytosis. Since the virus has no envelope, it cannot fuse with the plasma membrane.
After being engulfed, the viral nucleic acid is released from the endocytic vesicle. The nucleic acid then separates from the capsid.