Killer T cells launch an aggressive attack but don't last for long. Scientists are working on ways to draw a few back to a subdued memory form.
It does well to remember your enemies. When the human body encounters an infectious agent for the first time, it (often) successfully beats back the intruder by expanding in great number the immune cells that are specially equipped to clear out the pathogens and infected cells. After health is restored, a few "memory" versions of these immune cells remain, such that, should the infectious agent make a second unwelcome appearance, the immune response is quicker and more intense.