Epidemiology has its roots in public health. The first epidemiologists studied outbreaks of infectious diseases in Europe, such as smallpox and cholera, in the 1700s and 1800s. American epidemiologists began their work in the mid-1800s when yellow fever, typhoid fever, and influenza were problematic in large cities. These early researchers were usually physicians who were curious about why some people were more susceptible than others. They were the first to consider the role of inoculation, sanitation, and disease carriers. After reports in the late 1700s that dairy maids appeared to be immune to smallpox, an English physician, Edward Jenner, recognized that the maids were exposed to cowpox and that this dose of disease protected them. He developed a vaccine using the premise that the body would build an immunity if exposed to a small amount of a bacteria.2
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Cases of infectious diseases often cluster in neighborhoods or sections of cities before spreading. In 1854, John Snow plotted the home and work locations of cholera cases in a section of London and identified a single water pump as the source of the contamination.3 The story of Typhoid Mary provides a similar example. Outbreaks of typhoid fever in New York and New Jersey in the early 1900s were concerning because the known cause, poor sanitation, was thought to be fixed. An astute sanitation engineer traced the new cases to a single carrier, Mary Mallon. Mallon had worked as a cook in several homes where typhoid cases appeared, but she, herself, never got sick.4
Eventually, the record keeping that helped early epidemiologists make these connections was taken over by city health departments created to track disease and monitor conditions that led to outbreaks. The American Public Health Association was founded in 1872, and a permanent national Public Health Service was established in 1912.5 Records of births and deaths allowed health workers to find common causes of death and consider ways to reduce citizens’ risks of dying. Mortality data continue to be a large part of epidemiology. The major threats are no longer from the spread of bacteria or viruses, but from noninfectious diseases and accidents. Noninfectious diseases have no single cause, but they are related to lifestyle factors, such as tobacco use, physical inactivity, diet, or stress. In the past 50 years, epidemiology has advanced the knowledge of factors related to heart disease, diabetes mellitus, cancer, depression, and Alzheimer’s disease. Identifying common factors and knowing who is at risk of disease or injury is the first step in being able to reduce the number of people who develop illnesses or are injured each year.
Risk Factors and Statistics
A basic premise of epidemiology is that exposure to a disease agent increases the chances of getting the disease. The exposure idea is most obvious for infectious diseases (eg, influenza). The more contact one has with people that have the virus, the more likely one is to get sick. Exposure to a disease can also come from behaviors, the environment, or personal characteristics. Epidemiologists use statistics to quantify how much a disease or injury is related to behavioral, social, environmental, or personal characteristics. These links to disease and injury are called risk factors. Risk factors are conditions that increase the chances of being diagnosed with a disease or having a particular injury. They can be physiological, genetic, or inherited traits; health-related behaviors; or related to the physical surroundings.1 However, risk factors are not causes of disease; they simply have a connection to the disease. Based on the presence or absence of a particular risk factor, epidemiologists can calculate the probability, or risk, of developing a specific disease or injury. Several statistics are used to describe risk, such as relative risk, odds ratio, or population attributable risk. Also, life expectancy and disability-adjusted life years are statistics used by epidemiologists to show the impact of disease on longevity and quality of life. Chapter 3 explains many common epidemiology statistics in detail.
Epidemiology and the Leading Causes of Death
How strongly a risk factor relates to the onset of a disease or injury has a lot to do with the environment in which a person lives. Differences in health between groups of people are called health disparities. It is important to consider how known risk factors for disease are made worse by situations that are outside of an individual’s control. Some risk factors are changeable and are referred to as modifiable. However, for some people, there may be limits to how modifiable these risk factors are. Epidemiology helps make health care providers and policy makers aware of who needs the most help to change their risk factors. For example, smoking tobacco is a behavior that greatly increases the risk of heart disease and cancer. Smoking is more prevalent in minority populations and in those with lower levels of socioeconomic status. This knowledge has allowed health professionals to start smoking-prevention programs in elementary schools and has convinced lawmakers to increase taxes on cigarettes to make smoking less affordable. In terms of sports, differences in injury rates between genders, positions, and in different sports are risk factors that are made better or worse by larger circumstances, such as rules, playing conditions, and access to protective equipment.
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