A variety of factors contribute to the generation of memory or memory-like cells. In the mouse, CXCR6+ NK cells from the liver can mediate antigen-specific memory responses against haptens and viral antigens of VSV, HIV, and influenza via a yet-unknown receptor(s). During MCMV infection, the viral m157 protein is recognized by a subset of NK cells carrying the activating Ly49H receptor, resulting in the formation of m157-specific Ly49H+ memory NK cells. Memory-like NK cells in mice and humans can be generated by short-term stimulation with IL-12/15/18. A subset of human NK cells expressing the activating CD94/NKG2C receptor expands in response to the as-yet undefined antigens in HCMV, Hantavirus, Chikungunya Virus, HIV, and HBV infection.