In his book "Stopping The Clock", Dr. Ronald Klatz has listed his rules for Immortality. Rule #1 is: Don't Die.
And in case that doesn't cover the matter fully, Rule #2 is: Don't Get Sick.

Obviously, if we observe Rule #2 we will promote the observance of Rule #1.
And an important way of promoting the observance of both rules is to maintain a vigorous immune system.

With the advent of AIDS, almost everyone has by now heard of the immune system. The immune system consists of numerous entities in our bodies, chiefly components in the blood and the lymphatics, that protect us against disease.

If we look at some of the effects of AIDS on people - the wasting, the diminishing ability to fight off disease, we see that in some respects AIDS parallels aging, except that with aging, the effects manifest themselves over a longer period of time.

Likewise, Aging also brings on a general deterioration of the body. Accompanying this is a deterioration in the immune system, which in our later years, leaves us vulnerable to infections like pneumonia and flu. In our younger years, girded with a strong immune system, we usually fight these infections off within a week or two. But in our later years, with a weakened immune system, it is often the infections that win.