Evolution

Theodoius Dobzhansky said, "Nothing in biology makes sense except in light of evolution." The evolutionary origin of diseases, such as heart attacks, tuberculosis, myopia and cancer, has been one of the most mysterious, complex and controversial aspects in medicine. Darwin's evolutionary perspective on medicine, however, can help us understand the origin of diseases which can be a valuable input in the ultimate goal of medicine. Evolutionary explanation provides a better understanding of our bodies' malfunctioning, natural selection's inability to eliminate the genes that cause hereditary diseases and the causes of other infectious diseases. Natural selection for the most part is responsible for determining the existence of mild to lethal modern-day diseases. Since natural selection improves our chances of survival and reproductive success, modern-day diseases are perhaps associated with increased fitness.

Our immune system has been shaped over centuries to provide resistance to pathogens and ensure our survival. It is designed to attack, impair and label foreign particles, such as bacteria, dirt in the skin and cancerous cells. Most often after we recover from an infectious disease, our immune system becomes sensitive so that we have greater concentration of lymphocytes to destruct the pathogens that caused the particular disease later in life. However, our immune system is not perfect. It can sometimes fail to attack, expel or destroy lethal pathogens like tubercles bacterial cells, roundworms and other parasites while at other times, it attacks certain substances when it should not. Such mistakes increase the chances of our vulnerability to pathogens and contraction of deadly diseases.

We contract infectious diseases in an endless arms race against pathogens like bacteria and viruses. Both the host and the parasite evolve over time to be able to counteract each other's adaptations and ensure their survival and reproductive success. However, evolution does not proceed systematically and in order to make organisms better adapted, natural selection tries all sorts of adaptive means to make them fitter. It adapts organisms that can produce greater quantity of progeny better in successive generations. Since pathogens are reproductively more successful than us, they have a better chance of evolving against our defensive mechanisms to invade our bodies and cause diseases. For instance, though our resistance to some diseases like smallpox and tuberculosis has significantly improved over generations, the bacteria that causes these diseases have got even better adapted to counteracting our evolutionary impacts.

In a constant attempt to defend ourselves from the affect of pathogens, we have discovered antibiotics. However, over the course of years bacteria have gained resistance to antibiotics. Bacterial resistance to antibiotics does not arise by gradual development of tolerance towards antibiotics but by rare gene mutations or introduction of new genes through plasmids. In presence of antibiotics, resistant strains only thrive and if low levels of antibiotics are used later or if the use of antibiotics is stopped, besides the nonresistant strain, resistant bacteria also arise in high levels. By further evolutionary change, the resistant strain gets selected over the nonresistant one and dominates the bacterial population.

Pathogens not only try to defend themselves from the host's defensive mechanism but they also try to block the immune system's weaponry and proliferate their progeny so that they can nourish in their respective environments. In this way, pathogens have acquired adaptations to ensure their survival inside the hosts. For example, the human immunodeficiency viruses (HIV) bind to the surface of proteins on the cells of immune system and make the person with HIV vulnerable to cancer and other infections. Other viruses exaggerate expulsion mechanisms, such us coughing or sneezing, to spread themselves to other hosts. Therefore, an evolutionary understanding of the functional classification of signs and symptoms of diseases is important for figuring out if relieving a symptom would favor or delay the recovery of a patient.

In most cases, natural selection shapes adaptations such as expulsion mechanisms to enable us fight infections. Our bodies' defenses produce expulsion mechanisms, such as coughing and sneezing, to expel foreign particles or bacteria. Some symptoms of lethal diseases such as fever, headache, runny nose and malaise also ensure our recovery against infectious diseases.

Fever is an adaptation for conserving and regulating body heat to the normal level in order to fight contraction of infectious diseases rather than being a mistake in temperature regulation of the body. Studies on the effect of fever upon recovery from diseases have revealed that children who were given acetaminophen took a day longer to recover from malaria than those who took a placebo (sugar pill). These studies suggest that though we suffer due to symptoms of some disease, most symptoms are required to defend us against infections. Medications used to suppress them actually interfere with body's normal responsive mechanism to infections, often resulting in catastrophic consequences. Though evolutionary biology has clarified the adaptive significance of fever to the recovery from some diseases, in other cases the situation is not completely understood. For instance often it cannot be determined if fever is advantageous for the patient or the pathogen. It would be helpful if medical research is able to determine when fever is useful and when detrimental for us.

Like fever, iron withholding is also an adaptive mechanism to combat infectious diseases. Bacteria attack our immune system, use our nutrients to replicate in our bodies and find ways to spread. Iron supply is a critical and scarce source for bacteria. So, in order to prevent bacteria from thriving inside our bodies and spreading further, our bodies release a chemical that decreases the availability of iron. This adaptive mechanism can be observed in people who have tuberculosis. Such people have low level of iron in their blood to prevent tubercles bacteria from flourishing. Studies have shown that low iron level can help relieve infectious diseases. However, sometimes like fever iron withholding benefits pathogens rather the host. So, in order to know the affect of iron withholding on infections, further research should be conducted to study the association of pathogens with infectious diseases.

Natural section has shaped many mechanisms to help us avoid pathogens. For example, we tend to keep distant from patients with infectious diseases and take care of our hygiene to prevent breeding of pathogens. It has also enabled us to develop adaptations such as feeling pain to avoid danger and scratching when bitten