Final answer:
Animal viruses can penetrate host cells through receptor-mediated endocytosis or membrane fusion.
Step-by-step explanation:
Animal viruses can penetrate host cells through two primary methods: receptor-mediated endocytosis and membrane fusion.
Receptor-mediated endocytosis occurs when a protein in the viral capsid binds to its receptor on the host cell, leading to the virus being taken inside the cell via a vesicle during normal cell processes.
Membrane fusion occurs when the capsid proteins undergo shape changes after binding to the receptor, creating channels in the host cell membrane. The viral genome is then injected into the host cell through these channels.