IL2 and infection: We find that NK cells are more quickly and more extensively activated during nonlethal Py17XNL infection than during lethal PyYM infection, that this correlates with more rapid and more extensive upregulation of the IL‐2 receptor α‐subunit (CD25) and thus with expression of high‐affinity IL‐2 receptor, and that expression of CD25 and secretion of IFN‐γ by NK cells is partially dependent upon IL‐12, is augmented by IL‐2, and totally dependent upon signaling through the IL‐18 receptor.