The 6 Reasons Why Diseases Exist

6-reasons-diseases-existWhy do we get sick? Why aren’t organisms such as ourselves designed in such a way that they are resistant to disease? If you head into your GP’s office and ask your doctor about these things, he will likely tell you that there are no good answers to these questions and that each individual disease is unique in its etiology. He may then proceed to say that different diseases  develop for a variety of different reasons and that it’s impossible to categorize and organize all of these reasons into a short list that captures all of the fundamental causes of illness.

But is this really true? No… Actually, it’s not. There does actually exist a system that helps us make sense of why we get sick. This system is called evolution. Unfortunately, this system hasn’t yet made its way into the curricula of medical schools. The fact that many, if not most, health practitioners hold the types of beliefs outlined above isn’t surprising, given that conventional medical training largely focuses on the molecular mechanisms and proximate causes of disease, while paying little attention to the question of why diseases exist in the first place.

Randolph Nesse (PhD), one of the authors of the book Why We Get Sick: The New Science of Darwinian Medicine, has long made the case that there are 6 fundamental reasons why diseases exist (1). In today’s article, I thought I’d talk a little about each of these reasons, which are outlined below.

Six Evolutionary Explanations for Vulnerability to Disease (1)

Selection is slow
• Mismatch between design and environment
• Competition with a pathogen or other organism

Selection cannot solve some problems irrespective of time
• Tradeoffs
• Constraints peculiar to living systems, e.g. path-dependence

We misunderstand what selection shapes
• Traits that increase RS at the cost of disease vulnerability
• Aversive defenses are readily mistaken for diseases

Selection is slow

1. Mismatch between design and environment

All organisms on this planet are a product of evolution. Over billions of years, evolutionary forces such as natural selection have shaped the living world, sculpted it into its present form. Some species have died out along the way, whereas others have come into existence. The nature of these processes has largely been determined by the environmental conditions of life. Natural selection acts to adapt organisms to their environment. Those organisms that are poorly adapted to the milieu in which they find themselves tend to have a lower reproductive success than those organisms that are well-adapted to their environment. Hence, they pass on less of their genes to future generations. Over time, this causes changes in the gene pool of a population.

Traits that are adaptive in one environment aren’t necessarily adaptive in another. For example, if you live in a cold, arctic milieu, being covered in thick, warm fur is probably a good thing; however, if you live in a warm part of the world, near the equator, it obviously isn’t.

If the environmental conditions remain quite stable over time, the selective pressures tend to be weak. In other words, natural selection doesn’t have a lot to do. If the environmental conditions suddenly change however, it has to gear up. If the changes are rapid and/or profound, natural selection may have trouble keeping up. This is particularly true in the context of the evolution of macroscopic organisms.

This can result in an evolutionary mismatch, meaning that there’s discordance between the organism’s evolutionary design and its current environment. It may take a lot of time for natural selection to resolve this conflict, particularly if the organisms in question change their environment in such a way that the force of natural selection becomes reduces.

The term evolutionary mismatch is by some used exclusively to refer to a situation in which environmental changes cause maladaptation and reduced reproductive success. However, most of the time, the term is also used to refer to situations in which the outcome of the mismatch is ill-health and disease. Evolutionary mismatches often reduce both reproductive success and health. It’s important to note, though, that the two don’t always go together.

2. Competition with a pathogen or other organism

All organisms on this planet are part of an evolutionary arm’s race. If you fall behind in this arm’s race, you may quickly find yourself on the brink of extinction, due to the fact that other, more adept organisms start exploiting your weaknesses.

We humans have found ourselves in this situation many times throughout history. One of the primary threats to our species’ existence throughout our evolution has been microorganisms. Microbes evolve at a much more rapid pace than large, eukaryotic organisms such as us. Some microbes have evolved weaponry that we have a hard time defending ourselves against. Exposure to these types of microbes can result in disease and sometimes even death for the exposed person, if/she is not able to mount an adequate defense.

All of the life forms that are a part of a specific organism’s surroundings are a part of that organism’s environment. Hence, the situation above represents one type of mismatch between organism and environment.

Selection cannot solve some problems irrespective of time

3. Tradeoffs

In Norway, my home country, we have a saying that goes: “Du kan ikke få bade I pose og sekk”. It basically means something along the lines of “You can’t have it both ways” or “You can’t have your cake and eat it too”. This saying nicely explains what trade-offs are all about.

Let’s say that we follow a population of birds over many generations. Over time, we see that the male members of the population start developing larger, longer feathers and a more colorful plumage. By carefully examining what goes on, we elucidate that these changes likely occur as a result of sexual selection. Those male birds that carry a big, extravagant plumage seem to attract more females and get more offspring than males who have smaller, less extravagant feathers. Since the size, color, and shape of these birds’ feathers are largely determined by heritable biological information, this will result in changes in the physical appearance of the male birds over time.

In this situation, having a larger, more colorful plumage seems to confer elevated reproductive success. If it didn’t, we wouldn’t be seeing the changes we’re seeing.

However, having a larger, more colorful plumage is not unequivocally beneficial. The potential downsides are many. Predators will likely have an easier time spotting you, and the larger, heavier feathers may make you slower, less mobile, and more prone to developing joint pain and various other musculoskeletal issues.

In other words, in order to gain something, you need to lose something else. A trade-off occurs. This is how evolution works. You never get something for free. When you get better at one thing, you typically get worse at something else. You may also become more susceptible to developing certain types of health disorders, such as for example musculoskeletal pain or immune-related problems.

It’s important to remember that all organisms are composed of a bundle of traits. These traits are not separated from one another, at least not in the context of evolution. If one trait changes, others will likely change as well.

Here’s what Darwin had to say about this issue in his book On the Origin of Species:

The whole organism is so tied together that when slight variations in one part occur, and are accumulated through natural selection, other parts become modified. This is a very important subject, most imperfectly understood. (2)

4. Constraints peculiar to living systems, e.g. path-dependence

Natural selection can’t do magic. It can’t change something that isn’t there or create something out of thin air. It has to work with what it’s got.

The physical and biological structure of all the organisms that walk our planet today have been sculpted over billions of years of evolution. All of the changes that have occurred during this time have “accumulated on top” of all of the prior changes that have occurred.

Natural selection can’t suddenly decide to stop everything and reshuffle what has already been done. It has to live with what it’s done in the past and do the best with what it’s got in the present.

To illustrate this, let’s imagine that you own a car that you’ve been using for a couple of years. You’ve been quite happy with the car and want to keep it; however, you’re not 100% satisfied with its performance. For that reason, you take it into an automobile shop and ask them to improve the performance of your vehicle: make it faster, more stable on the road, and more durable.

The workers at the automobile place tell you that they can certainly help you out, but that that many of the components and structures that are already in place put constraints on what it’s possible to do, and that it will be very difficult to dramatically improve the performance of the car without taking out and replacing most of its parts. In other words, there are confines with regards to what can be done. Unless the mechanics tear out everything and start from scratch, there’s a limit to what types of changes they can bring about.

The concept of biological constraints is closely related to trade-offs, in the sense that there are constraints with regards to what selection can do without causing more “harm” than “good”. Sometimes, the pros of doing something outweigh the cons. For example, a significant change in the musculoskeletal system of a species may make the members of that species faster and better adept at running long distances; however, it could also make them worse at climbing and more prone to develop a variety of musculoskeletal disorders that reduce evolutionary fitness.

The fact that that there are constraints with regards to what natural selection can do and that there’s an unpredictable/random element to evolution (i.e., some useful alleles will be lost and some disease alleles will persist) helps explain why diseases exist.

We misunderstand what selection shapes

5. Traits that increase RS at the cost of disease vulnerability

A lot of people misunderstand what natural selection does. Unlike what some people believe, natural selection doesn’t select for health. Rather, it selects for reproductive success. Often, the two are linked, but not always. For example, some chronic diseases develop primarily late in life and have little impact on reproductive success.

In some instances, there can even be an inverse correlation between reproductive success and certain health parameters. For example, those ancestors of ours who carried lactase persistence alleles got on average more offspring than those who didn’t (particularly during times of scarcity), due to the fact that they were able to consume milk and other lactose-containing dairy foods. Consuming these types of foods was beneficial in terms of survival and reproduction, as they provide quite a bit of nutrients and calories. However, it may have been detrimental to long-term health, in the sense that regular consumption of milk and other dairy foods may impair protection against a variety of degenerative diseases.

6. Aversive defenses are readily mistaken for diseases

Sometimes, we mistake evolved defenses for diseases. For example, we may fail to acknowledge that fever, diarrhea, vomiting, and other symptoms we might experience when we get sick aren’t necessarily bad things that should be avoided or eliminated, but rather evolved responses to infection and disease that help the body recover.

For example, a rise in the temperature of the body can make it harder for pathogenic life forms to spread and reproduce. The evolutionary fitness of microorganisms is largely affected by temperature. Just a small drop or rise in the temperature may significantly affect a pathogenic bacterium’s capability to survive and reproduce.

The fact that we have evolved various defense systems that help us cope with illness and infection is very important to recognize, as it can help us understand what constitutes the best course of action for dealing with a variety of different ills and pains. This in turn will translate into improved health care.


  1. You might enjoy Prof stearns lecture modules on evolution and evolutionary medicine both available free on Yale’s YouTube channel.

  2. Thanks for explaining the constraints point.
    Evolutionary branches are like Markovian processes where the likelihood to be in a state depends on the previous one.
    Some people misunderstand how evolution work and just think about time overlooking the path of a species.
    It’s not granted to achieve the X mutation at time t that allows a species to cope with the environment. It’s a hugely complex function of likelihood and some kind of randomness.


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