If human life is so long largely due to modern medicine, does every illness shorten lifespan?

Question

One of the most often-cited facts about human life, compared to those of other animals, is that the main reason we live so much longer is modern medicine. Because we can treat illnesses that would previously affect lifespan, we are far more likely to live greatly extended lifespans. However, this leads to two possible (conflicting) logical conclusions:

1. People who by chance didn't get deadly diseases before modern medicine would live as long as people today, meaning the ability for any individual to survive ninety or more years, far longer than nearly all animals, is unrelated to modern medicine.
2. Every illness one experiences weakens the body in some way, robbing it of future years. This would mean the role of modern medicine in extending lifespan is treating these illnesses to prevent the gradual reduction in lifespan.

If the first is true, then lifespan itself isn't influenced by modern medicine unless it prevents death as the direct result of a disease, and only average lifespan is affected. In other words, if nine in ten dies at age thirty due to a deadly disease, and one in ten dies at age eighty by avoiding disease, the average life expectancy is thirty five, even though an individual could by living an extremely careful life survive to reach eighty.

If the second is true, then short periods of non-deadly illnesses experienced by everyone each shorten life expectancy by a tiny amount, together decreasing everyone's lifespan to the same thirty five, rather than the effect being a result of averages.

So does each illness shorten lifespan, or is it only a result of averages that lifespan was so low pre-modern medicine, and humans always had the capacity for exceptionally-long lives?

Answer 1

Both. Human cells largely exhibit a phenomenon called senescence - they just give up and die after they reach a certain age via a biochemical mechanism called apoptosis. The outer limit of survivability for human cells is generally understood to be in the 100-120 year range. One of the things that makes a cancer cell cancerous is the deactivation of the signals for apoptosis, making the cell effectively immortal.

At the same time, most illnesses, particularly the "lifestyle illnesses" (diabetes, hypertension, hypercholesterolemia) so common today, do induce some changes which shorten lifespan.

The most significant changes which extended human life were the development of (a) sanitation and (b) antibiotics. These together dramatically reduced death by infectious disease. Removal of early death from infection exposed these diseases of late-life and lifestyle.

Answer 2

The claim of modern medicine being responsible for longer lifespans is of course a statistical claim, i.e. the average/net effect. Thus your second assertion does not follow logically. Though it is an interesting point.

Diseases, both now and in the past, do not have 100% mortality rates. If you followed the ebola outbreak you'd have heard stories of survivors are quite common. The same is true with measles, the common cold, lung infections, etc. When we find a cure for a disease the lifespan statistic increases. This is because those people who would have died survive instead, NOT because infected people avoid long-term collateral damage that would have taken a few years of their 'original' lifespan. At least that is the current predominant view of things.

Consider the following:

• Diabetes decreases individual lifespan, ergo the average lifespan also decreases if enough people get diabetes.
• The common cold does not decrease individual lifespan, yet it still contributes to a lower average lifespan because it may be fatal in weak individuals.

So we could say (almost) every illness shortens the average lifespan, but not every illness shortens individual lifespan.