Final answer:
Cytotoxic T cells and NK cells use perforin to create pores in target cells' membranes, allowing granzymes to enter and induce apoptosis, thus controlling infections and preventing cancer progression.
Step-by-step explanation:
The mechanism through which cytotoxic T cells and NK cells execute a "lethal hit" on infected or transformed cells involves a protein called perforin. Perforin is a destructive protein that creates pores in the membrane of the target cell. Once these pores are formed, granzymes, which are proteases, enter the infected cell through these pores and induce programmed cell death, or apoptosis. This process ensures the controlled destruction of the target cells, thereby preventing the spread of infection or the growth of cancerous cells.