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The human body is a marvel. Factually, the human body consists of a head, a neck, a torso, two arms, and two legs. More awe-inspiring is the fact that the human body is a complex integration of several congruent systems that work in synchronicity to form the most complex multicellular organism. It is constantly changing and adapting. Evolution's application to medicine has changed the way in which we view and think about the human body, and the diseases that affect it. Applying evolutionary considerations to medicine has greatly impacted our view of diseases, the human body's defenses against diseases, and made us ask why natural selection has left us vulnerable to disease?
The principle of Natural Selection states that whenever there is an inheritable variation in some trait that influences reproductive success, the trait will inevitably change over the generations.  Simply, if within a population there exists genetic variation with certain traits being more successful than others then, over time, the genetic makeup of the population will inevitably change. The principle of natural selection has been greatly debated; however, the consensus is that the imperfections present within the human body serve a purpose and are not a result of natural selection being too weak. Rather, natural selection shapes traits for genes not the species. It shapes the body to maximize reproductive success and, therefore, plays a key role in humans, as a mammalian species, achieving its basic biological goals of survival and reproductive success. Evolutionary explanations are based upon natural selection and attempt to describe how certain traits increases fitness - that is, why these traits exist in the forms that they do.
The eye is considered to be an organ of extreme perfection and a perfect example of natural selection selecting for fitness, while at the same time leaving the eye vulnerable to disease. It possesses several physical characteristics that heighten its function, as well as characteristics that function as defense mechanisms that protect it. For example,
the only clear tissue in the body is in front of the cornea; the lens is flexible so that eye can focus from near to far; and tiny fibers are found within the eye that help to squeeze it in the just the right way and time. The iris expands or contracts to let in more or less light and even the eyelid itself serves as a defense mechanism - shutting quickly when anything approaches rapidly. And tears that steadily coat the surface of the eye, floating from the outer edge to the inner edge to constantly keep it clean. However, the eye is also, the exemplar of natural selection's limits since all of these perfections are accompanied by a susceptibility to such imperfections as glaucoma, myopia, cataracts, presbyopia, iritis, the cornea can become clouded, and the retina can detach. 
Common genetic disease is often a result of the interactions between genes that are considered to be harmless until they react with certain environments. Based upon this assertion, evolutionary explanations claim that it is not disease that is shaped by selection, but the vulnerability to disease that is. Natural selection can help explain maladaptation or imperfections present within the human body as well as adaptation by explaining why the body isn't better.
This begs the question, however, of why do all members of a species share certain traits that leave them vulnerable to disease? Why has natural selection left us vulnerable to disease? As Nesse outlines there are several reasons why diseases exist. According to Nesse, natural selection is just too slow, which results in a mismatch or lack harmony between or bodies and our environment  - a physical and socio/cultural environment that we have created ourselves. Further, our lifestyles today greatly differ from that of our ancestors and our bodies are not designed to be compatible with our current sedentary lifestyle or a high-calorie diet, leaving us vulnerable to many diseases. The human body has "only recently adopted its current environment."  With different fast food restaurants in abundance and a substantial decrease in the amount of exercise per day, the human species, and in particular North Americans, are increasingly becoming more and more overweight and susceptible to obesity, cancer, and other modern diseases. Our bodies were originally designed to consume a limited amount of fats, salt, and sugar and exercise hard each day, but with the increasing power of marketing and advertisements, cultural norms and priorities have evolved and affected our intellect. The majority of the human population believe that the more you have, the more power, status, and money you are perceived to have, which is the ultimate goal in our modern society. Humans have, therefore, changed their behaviour based on this novel environment without taking into account the adaptability and design of their bodies which has resulted in an increase in their susceptibility to diseases.
Further, there is competition between organisms, such as pathogens, that evolve much quicker than our bodies, often leading to an arms race between the body, or host, and the parasite that has infiltrated the body. Williams describes infection as the body's response to an encounter with another organism. The response is often one of an arms race between the body, or host, and the parasite that has infiltrated the body. One of the body's evolved defenses against infections is that of fever. A fever is not just an increase in metabolic rate; it is the carefully regulated rise of the body's temperature.  This rise in body temperature kills any pathogens that are considered to be threats to the well-being of the body.  A fever is a protective adaption that is regulated by specialized regulatory mechanisms within the body, which have evolved in response to bacterial toxins. Unfortunately, in response to the defensive mechanisms that natural selection has shaped within the human body, pathogens quickly evolve by developing strategies that enable them to evade or thwart the body's defenses or they face extinction. Our response to these strategies have been the use of drugs or vaccines, and while we have won huge battles in the war against pathogens with these 'weapons', many vaccines are imperfect and are unable to completely and permanently eradicate a pathogen from the body of a vaccinated person, which could lead to an increase in the virulence of the pathogen or even antibiotic resistance.  Further, in the microbial world, there is a vast amount of interspecies competition for space and food sources. Microbes are aggressive and when faced with negative natural selection in the form of antibiotics, they evolve and develop a genetically transmitted resistance to individual antibiotics and sets of antibiotics. This resistance to antibiotics then makes them pathogenic to humans, which instigates an "arms race" between humans and these pathogens with respect to antibiotics.  Multiple-drug-resistant TB myobacteria, in particular, has become one of the most threatening diseases and issues in public health during the past 20 years. This issue is especially distressing amongst patients who have HIV/AIDs who already have impaired immunological systems.
In addition, we misunderstand that organisms are not shaped for health or longevity, but for maximizing reproductive success. If a gene increases reproductive success, it will spread even if it makes life shorter. Finally, there are defenses and suffering. For example, pain, nausea, fear, vomiting, and fatigue seem like medical problems, but are in fact protective responses that have been shaped by natural selection and whose expression is regulated. They are expressed whenever it is to the benefit of the body. They are not diseases, rather they are symptoms of diseases and the distinction is very important.
A better question, however, is "are the defenses of the human body to disease useful?" From an evolutionary perspective, defenses against diseases, such as fever, cough, pain, and anxiety are not design flaws, but are instead evolved defenses. For example, coughing is one of the most useful defense mechanisms the human body has in its arsenal since the failure to remove foreign matter from the lungs can lead to a person to die of pneumonia.  Further, the capacity for pain is highly beneficial and has an important purpose. Pain is a sensory or emotional experience, varying in intensity, that prompts humans to withdraw from potentially damaging situations and protect their bodies from painful stimuli. There are some people who do not experience pain. This insensitivity is rare and disadvantageous. These individuals often do not feel the discomfort that normally occurs when they stay in a prolonged position. Being unable to feel the stiffness and pain that occurs from such an occurrence can weaken the amount of blood being pumped and supplied to their joints, causing them to deteriorate and eventually leading to premature death due to tissue damage and infection. 
Another evolved defense maintained by the human body includes the presence of low levels of blood iron. A reduced level of iron in the blood is often a symptom of a chronic infection and is the body's protective mechanism to an infection. The body will redirect and restrict iron to the liver as a way of depriving bacteria of this element. 
Public health responses have led to many pathogens increasing in virulence, however, as Omenn advocates, treating infectious diseases or counteracting the spread of pathogen does not necessarily need to result in an arms race. Omenn purports that vaccines and treatments can be developed so that they trigger less virulence and more desirable traits are selected. One of the examples he puts forward is that of the diphtheria toxoid vaccine. This particular vaccine selects against toxin production, the cause of the disease, instead of other characteristics of Corynebacterium.  Thus, diphtheria infections still occur, however, the extant strains now lack toxin production. This is a significant development and improvement in public health since one instance of immunization is essential to the prevention of such an infectious diseases.
Our understanding of evolution and its application to medicine have greatly impacted our view of diseases and our public health responses to diseases. The evolutionary mechanisms that occur in response to diseases are no longer viewed as flaws, but instead as the human body's evolved responses to disease. Further, having a better understanding of evolution and a greater application of evolution to medicine has affected public health measures so that vaccinations are being developed to select for less virulence of pathogens in order to circumvent an arms race between the human body and pathogens that are considered to be a threat.