For example, lymphocytic choriomeningitis virus (LCMV)-specific memory CD8 T cells are maintained at stable numbers in the spleen for the life of the laboratory mouse [18], and detectable numbers of memory CD8 T cells can be found in human PBL 20–75 years after natural exposure to, or vaccination against yellow fever virus, measles virus, and smallpox [19–23]. Here, CD8A is linked to smallpox.