An Example of the Damage Done
From Immunity & Aging, one PDF-format example of the way in which the cytomegalovirus-compromised aging immune system lets you down: natural killer (NK) cells "are a component of nonspecific immune response involved in the defense against viral and bacterial infections ... infections and pathogen burden have been considered as a potential risk factor for [coronary heart disease (CHD)]. The main suspects linked to the vascular disease are wide-spread organisms such as cytomegalovirus (CMV) and Chlamydia pneumoniae. ... as infections deteriorate the course of CHD and NK cells are the first line of defense against infections, there might be a link between development of CHD and NK cells. ... These data indicate that an active infection with specific pathogens
may affect NK cells functions. Vice versa, low NK activity may be one of the possible gates of spreading infection, and this may be a way of deepening of NK cell deficiency. Altogether, it creates a positive feedback loop leading to accelerated progress of diseases associated with chronic infections." When your system starts to run down, the process of degeneration accelerates - but the initial accumulation of damage is gradual, which is why the development of repair strategies can be a path to greatly extending healthy life span.
Link: http://www.immunityageing.com/content/pdf/1742-4933-4-3.pdf