Answer:
Enter the host cell.
Step-by-step explanation:
Viruses attach themselves to the cell surface receptors present on their host cells. Binding of viruses to these receptors is followed by penetration of the host cell.
For example, the flu virus binds to the cell surface receptors located specifically on the surface of cells of the respiratory system. Hence, the cell surface receptors of the host cell are used by viruses to enter the host cell.