The follow abstract is taken from Fightaging.org.
The environment surrounding our tissues and various complex systems such as organs incorporates a great deal of microbial life. We are surrounded by microbes, we have a whole cooperative ecosystem on our skins and another in our guts, and are constantly under attack by less friendly species. From the point of view of a great many classes of microbial life, we mammals are just another resource to be exploited as a basis for unfettered replication. Before the advent of modern medicine, life expectancy was largely determined by infectious disease and other environmental pathogens rather than the fundamental processes of aging. In the research paper linked below, the author makes a valid point, which is that we haven't really yet defeated the hostile microbes arrayed against us, just postponed their inevitable victory by decades for most individuals. When we consider aging, we should think about aging in the context of our vulnerability to the microbial world in addition to the failure of our component parts for other reasons.