Necessary and Sufficient Cause of Disease

Necessary Cause of Disease

 A necessary cause of disease is always associated with a particular outcome because an exposure is required for a particular outcome. For example; HIV is a necessary cause of AIDS. An individual cannot be diagnosed with AIDS if they are not infected with HIV.

Sufficient Cause of Disease

A sufficient cause of disease is an exposure that may produce a particular outcome but may not be the only cause of that outcome. An example would include poor nutrition leading to obesity, however, sedentary lifestyle, stress and other factors could have also caused the disease.  

http://minorityhealth.hhs.gov/templates/content.aspx?ID=3024

According to the US Department of Health and Human Services, American Indians and Alaska Natives are twice as likely to develop diabetes than non-Hispanic white individuals. Risk factors associated with diabetes include hypertension, obesity, high cholesterol and tobacco use. Biologically, American Indians/Alaska Natives and non-Hispanic white individuals are equal. Non-Hispanic white individuals have not developed an immunity to hypertension or obesity. In fact, risk factors such as high cholesterol, hypertension and obesity plague every society, race and ethnicity. So why are certain ethnicity's more susceptible to diseases than others? 

Simply stated, social determinants of health impact disease prevalence. Social determinants of health include; gender, income level, place of residence, age, occupation, education level and socioeconomic status. The documentary series Unnatural Causes explores the topic of inequality and its effect on health. In the first episode, In Sickness and In Wealth, investigates the connection between physical health, wealth and race. After interviewing several individuals in different income brackets, they discovered that the individuals with the highest income were the most healthy. Reasons include; access to better housing, more comprehensive healthcare, better transportation and increased availability of healthy foods. Along the health-wealth gradient, they observed 5 to 10 year gaps between the cities rich, middle and working class neighborhoods. As indicated in this documentary series, social determinants of health can be a sufficient cause of disease. Individually they may not produce a particular outcome but collectively, determinants such as poverty, poor nutrition and increased stress can impact an individual's health. 

There are several determinants that play a role in the increased prevalence of diabetes in minorities including lack of proper nutrition and increased stress levels. American Indians, especially those living on reservations, are also challenged because of their place of residence and occupation. American Indians have typically made a living off growing and harvesting crops. Over the recent decades, water sources and irrigation systems have been rerouted in the mid-west to better accommodate bigger farms that produce more crops. Consequently, American Indians harvest less crops and therefore have minimal access to nutrient dense food. In an effort to preserve American Indian culture, the government has created  subsidized housing  and provided food to various reservations throughout the region. However, the food that they are providing is high in fat, sodium and cholesterol. As a result, there has a been a rise in obesity and diabetes among American Indians. In addition, there is a lack of health education and health promotion on reservations. This exemplifies the impact that occupation and place of residence can have on an individual's health. 

Globalization and Infectious Disease

Today, diseases as common as the cold and as rare as Ebola are circling the globe with near telephonic speed, making long-distance connections and intercontinental infections almost as if by satellite. You needn’t even bother to reach out and touch someone. If you live, if you’re homeothermic biomass, you will be reached and touched.

—Angier, 2001

Globalization

Pros: 
- Free trade promotes global economic growth, creates jobs and lowers prices for consumers. 
- Worldwide market for companies and consumers to access products from different countries.
- Exchange of information and culture between countries.  
- Cooperation among countries lead to ecological, environmental and financial well-being.
- Increased tolerance and open to different world views. 

Cons: 
- Exploitation of developing countries; social injustice, unfair working conditions, mismanagement of natural resources and ecological damage.
- Multinational corporations have increasing political power and less accountability 
- Americanization and spread of homogenous culture. 
- Human trafficking of men, women and children for both sex and labor. 
- Increase in the spread of communicable diseases. 

Globalization has made a lasting impact on the state of the global economy and global health. At any given time, there is a wealth of untapped knowledge just a plane flight away. However, with an increase of global travel and migration, we have witnessed an increase in the spread of infectious diseases. 

Globalization has led to overcrowded metropolis areas. 

Spread of Infectious Disease

The global spread of infectious disease is more complex than an international traveler being exposed to a tropical illness. Globalization has increased the large scale movement of goods across the world. An increase in global trade has led to the introduction of carrier vectors into unexposed environments. For example, global trade led to the reemergence of cholera in South America in 1990's. A freighter from China is thought to have expelled ballast water that contained algal blooms contaminated with cholera into South American coastal waters (Pictured below). Individuals consumed fish and crustaceans that had filtered the contaminated algea. Once individuals were infected, the disease spread rapidly across South America, fatally infecting thousands of individuals. 

Example of a freighter expelling ballast water. 

Public health professionals continue to work tirelessly towards the eradication of diseases such as malaria, yellow fever and AIDS within Africa. Not only are they concerned with migration between countries but migration within countries as well. Globalization has led to an increase in migration to urban areas due to an increase in available jobs and hope for a better lifestyle. In Africa, yellow fever is primarily found in rural areas but globalization has instilled a sense of fear that the virus could spread to more highly populated areas. Epidemiologists have been tracking the disease and sampling mosquitoes from various regions to identify patterns and predict the next outbreak. 

Epidemiological map tracking the spread of H1N1 disease. 

What is epidemiology?

Epidemiology in its most basic definition is the study of patterns of disease within a given population. In addition to patterns, epidemiologists study the cause and effect that a disease has on a population. When I think about epidemiology, I think of the 1854 Broad Street cholera outbreak. John Snow was one of the first individuals to propose the germ theory of disease. The germ theory of disease suggests that diseases are caused by microorganisms that grow and reproduce within host organism. The germ theory of disease replaced the concept of "bad air." After talking to community members, Snow deduced that the source of the Cholera outbreak was the water pump on Broad Street. In a time where microorganisms had not been discovered, Snow was able to find a pattern of disease and locate the source. Below is a picture of the clusters of Cholera within Soho, London in 1854. His innovation has led the way for modern medicine. 

Although epidemiology is rooted in infectious disease, it also explores chronic disease and conditions such as diabetes, cancer, pregnancy and injury. The word epidemiology comes from the Greek words epi meaning "on", demos meaning "people" and ology meaning "the study of." Epidemiology relies on statistical analysis to calculate frequency and distribution of disease within a population. What interests me about epidemiology is that there is a greater focus on population health rather than individual health. I believe that a person's health is largely determined by their community. Intrinsic controls play a crucial role in lifestyle behaviors but extrinsic controls such as stress, poverty, and society can define an individuals health status. Epidemiology focuses on big picture and determines the cause of the disease or condition. For example, an individual can be treated for heart disease by a physician but an epidemiologist can study an entire population to identify the prevalence and cause of heart disease. That epidemiologist may find that there is a lack of physical education within the community and promote physical activity. In a way, epidemiology is a type of preventative care. If we can understand the cause of a disease or condition, we can better prepare that community for prevention.