There are three major factors that affect human health: the environment, genes, and lifestyle. The environment includes not only physical surroundings but also social and political conditions. Genes form part of our DNA, which is a molecular structure in every cell that carries hereditary information from one generation to another. Lifestyle refers to all activities we practice, consciously or otherwise, including diet and exercise habits.

The environmental factor of human health is the surroundings and conditions of where we live. These include physical surroundings such as air and water quality, and social and political conditions such as poverty, war, and pollution. The environment influences many factors that affect our health, some more obvious than others. For example, it is clear how air pollution affects our lungs, which in turn affects our ability to breathe. However, it is not as obvious how social factors such as poverty and war affect the risk of developing a disease through stress hormones.

Numerous studies have been done on how physical surroundings affect human health. In one study by the University of Illinois at Urbana–Champaign Department of Natural Resources and Environmental Sciences, elderly patients were exposed to two different scents while the researchers monitored their health. The first scent was a floral scent and the second was a combination of grass and tree scents. Researchers found that after exposure to either scent, there was an overall improved response in the patients' immune systems. These findings were specifically related to one type of cell called T cells which increase in number when we fight disease or infections.

The social factor of human health is the social conditions such as poverty and war that affect our bodies through stress hormones such as cortisol and adrenaline. However, this factor also includes other factors such as lifestyle choices like smoking tobacco or drinking alcohol excessively, which can lead to diseases such as cancer or cirrhosis of the liver.

Studies have shown that poverty increases the risk of developing chronic diseases like heart disease, cancer, and diabetes. A study published in 2008 by New England Public Health indicated that poverty leads to lower levels of education which leads to less knowledge about healthy habits such as smoking, eating healthy foods, and following screening guidelines, all of which are preventative measures for many illnesses.

Another factor related to social conditions is war. War brings destruction not only through physical violence but also through psychological damage leading to stress hormones being released at high rates. Such stressors have been found to impair sleep quality which is associated with poor mental health and injury recovery time, according to Stanford University's School of Medicine.

Lifestyle choices are things we do either consciously or unconsciously that can affect our health. These include smoking tobacco, drinking alcohol excessively, and consuming high sugar diets. Smoking is one of the most well-known factors for health problems like cancer and heart disease because it leads to an increased risk for lung cancer as well as chronic obstructive pulmonary disease (COPD).

Other lifestyle choices can be less obvious at first but still potentially deadly. Alcoholic beverages are associated with numerous types of cancers such as mouth, throat, larynx, liver, colon, rectum, breast cancer in women; however, not all people who consume alcoholic beverages will develop these cancers. One theory why excessive alcohol consumption increases cancer risk relates to how alcohol metabolism creates acetaldehyde which forms adducts with DNA that lead to mutations that result in cancer.

As for excess sugar diets, a high intake of sugar is associated with an increased risk of obesity, type 2 diabetes, and cardiovascular disease which are all leading causes of death in the United States. An example of how this works is having a lot of sugar readily available increases hunger and reduces satiety so we may crave food and eat more than we normally would if we felt full. Furthermore, the excess energy from the sugar needs to be eliminated so it enters our bloodstream and goes into our cells to build molecules such as glycogen or triglycerides which can lead to insulin resistance over time. Resistance means that cells don't respond well anymore to insulin signaling such that there will be a large amount of glucose present in the blood. This is called hyperglycemia which can lead to diabetes which causes problems of its own such as an increased risk of heart disease and stroke.

A final social factor that affects human health is our level of education. Although education doesn't determine health on its own, it does impact how we make choices that might affect our health such as smoking or drinking alcohol excessively. A study published in the World Health Organization's (WHO) International Journal for Equity in Health showed that people who had at least some college were half as likely to smoke, twice as likely to exercise regularly, and 25% less likely to be obese compared with people who had never been to school or only received a primary level education.