Final answer:
Viruses penetrate host cells primarily through engulfment known as endocytosis, or by direct fusion when the viral envelope fuses with the host cell membrane.
Step-by-step explanation:
The 2 principal processes by which viruses penetrate host cells are engulfment or endocytosis, and direct fusion. During penetration, a virus enters a host cell either by being engulfed through endocytosis or by the viral envelope fusing with the host cell's plasma membrane through a process known as membrane fusion.
Receptor-mediated endocytosis is common among many viruses, enveloped or non-enveloped. However, direct fusion, which requires the virus to have an envelope, allows for the viral genome and capsid to enter the host cell cytoplasm directly. This process is facilitated by fusion proteins found in the viral envelope, such as those in the HIV virus, which merge with the cell membrane and release the virus's nucleic acid into the cell.