Final answer:
Endocytosis is the process where viruses are engulfed by a host cell and taken in via a vacuole or vesicle. Receptor-mediated endocytosis and fusion are two ways viruses enter a cell, with the former including both enveloped and non-enveloped viruses and the latter involving enveloped viruses only.
Step-by-step explanation:
The process of endocytosis, a type of penetration, occurs when viruses are taken into the cell followed by engulfment in a vacuole or vesicle. During receptor-mediated endocytosis, the cell membrane surrounds and engulfs the entire virus, which is then internalized within a vesicle. This can happen with both enveloped and non-enveloped viruses, where the viral capsid or the entire virus is brought into the host cell. Some enveloped viruses utilize another method called fusion, where their envelope merges with the host cell's membrane, allowing the viral genome to enter the cytoplasm directly.