Well, according to scientists at the Stanford University School of Medicine our stem cells – which create
the cells that comprise the blood and immune system – are not immune to the aging process. The
researchers, who studied mice and people, say stem cells begin to falter as we age and this could explain
why our chances of getting some diseases, such as acute myeloid leukemia, increase with age, and could
also explain why elderly people tend to be more vulnerable to infections such as colds and the flu.
 

Read more: http://www.sciencedaily.com/releases/2011/11/111128152414.htm