Assuming that you fear getting sick and dying, you really ought to think more about Mismatch Conditions. They are also a fundamental evolutionary process.
Mismatch conditions are problems, including illnesses, that are caused by organisms being imperfectly or inadequately adapted to novel environmental conditions. As extreme examples, a chimpanzee adapted to the rainforests of Africa would be hopelessly mismatched in Siberia or the Sahara, and a hyena would be mismatched to a diet of grass or shrubs. Such radical mismatches almost always cause death and sometimes cause extinction.
Mismatches, however, are typically more subtle and most commonly occur when climate change, dispersal or migration alters a species’ environment, including its diet, predators, and more. Natural selection occurs when heritable variations to these sorts of mismatches affect offspring survival and reproduction. For instance, when tropically adapted humans who evolved in Africa dispersed to temperate habitats such as Europe about 40,000 years ago, selection acted rapidly in these populations to favor shifts in body shape, skin pigmentation and immune systems that lessened any resulting mismatches.
Although mismatches have been going on since life first began, the rate and intensity of mismatches that humans now face has been magnified thanks to cultural evolution, arguably now a more rapid and powerful force than natural selection. Just think how radically our body’s environments have been transformed because of the agricultural, industrial and post-industrial revolutions in terms of diet, physical activity, medicine, sanitation, even shoes. While most of these shifts have been beneficial in terms of survival and reproduction, everything comes with costs, including several waves of mismatch diseases.
The first great wave of mismatches was triggered by the origins of farming. As people transitioned from hunting and gathering to farming, they settled down in large, permanent communities with high population densities, not to mention lots of sewage, farm animals and various other sources of filth and contagion. Farmers also became dependent on a few cereal crops that yield more calories but less nutrition than what hunter-gatherers can obtain. The resulting mismatches included all sorts of nasty infectious diseases, more malnutrition, and a greater chance of famine.
A second great wave of mismatch, still ongoing, occurred from the industrial and then post-industrial revolutions. The standard description of this shift, generally known as the epidemiological transition, is that advances in medicine, sanitation, transportation, and government vastly decreased the incidence of the communicable diseases and starvation, thus increasing longevity and resulting in a concomitant increase in chronic non-infectious diseases. According to this logic, as people became less likely to die young from pneumonia, tuberculosis or the plague, they became more likely to die in old age from heart disease and cancer—now the cause of two out of three deaths in the developed world. The epidemiological transition is also thought to be responsible for other diseases of aging such as osteoporosis, osteoarthritis and type 2 diabetes.
The problem with this explanation is that aging is not a cause of mismatch, and we too often confuse diseases that occur more commonly with age with diseases that are actually caused by aging. To be sure, some diseases like cancers are caused by mutations that accrue over time, but the most common age of death among hunter-gatherers who survive childhood is between sixty-eight and seventy-eight, and studies of aging among hunter-gatherers and subsistence farmers routinely find little to no evidence of so-called diseases of aging such as hypertension, coronary heart disease, osteoporosis, diabetes, and more. Instead, these diseases are mostly caused by recent environmental changes such as physical inactivity, highly processed diets and smoking. In other words, they are primarily novel mismatch diseases caused by industrial and post-industrial conditions.
In short there are three reasons you should pay attention to the concept of mismatch. First, mismatches are a powerful evolutionary force that always have and always will drive much selection. Second, you are most likely to get sick and then die from a mismatch condition. And, most importantly, mismatches are by nature partly or largely preventable if you can alter the environments that promote them.