The ability of the immune system to protect the body from various insults declines with age. Changes in both innate and acquired immunity appear to be involved in this phenomenon, and they can be described at the level of the entire organism or the cell. For example, the innate response of mounting a fever with infection occurs less frequently in aged persons; 20% of adults over age 65 with serious bacterial infections do not have fevers. This may be due to decreased sensitivity of the central nervous system to immune signals, rather than actual changes in the ability to generate such stimuli. Most changes in immune function associated with advancing age have been observed at the cellular level and involve acquired immunity.