Introduction to Epidemiology

• Epidemiology is the study of populations to determine causes of health and disease
• Epidemiology focuses on the distribution and determinants of disease frequency in human populations

About Epidemiology

• Charles-Edward Amory Winslow called epidemiology "the diagnostic discipline of public health" in the early 20th century
• Epidemiology is used to perform the public health assessment function

Epidemiology Defined

• Epidemiology relates to epidemic and derives from a Greek word meaning "upon the people"
• An epidemic is an increase in the frequency of a disease above the usual and expected or endemic rate
• Epidemiologists count cases of illnesses, and ask who is getting the disease, and also where and when the disease is occurring
• Epidemiologists make informed guesses to why diseases are occurring
• A core goal of epidemiology is to control and prevent the spread of disease

How Epidemiology Works

• Epidemiology was used to study and control a disease in London between 1853-1854, and it illustrates how epidemiology works
• John Snow, a British physician, is considered the father of modern epidemiology
• Snow was concerned with a cholera epidemic in London in 1848
• Death rates were high in areas where water was supplied by two private companies
• Both companies drew water from the Thames River at a point highly polluted with sewage
• The Lambeth Company changed its source to an area of the Thames free of pollution in 1849-1854
• Cholera deaths decreased in the Lambeth Company area, while no change occurred in the Southwark and Vauxhall Company areas
• Snow hypothesized that cholera was spread by polluted drinking water

Work of Snow

• A severe outbreak of cholera concentrated in London’s Broad Street area in 1853
• Some houses were supplied by one water company, some by another, permitting a natural experiment
• People of both sexes, of every age and occupation, and of every rank and station... were divided into two groups without their choice, and, in most cases, without their knowledge."
• Snow visited each house where someone died of cholera to determine which company supplied the water
• In 40,046 houses supplied by the Southwark and Vauxhall Company, 1263 cholera deaths occurred
• In 26,107 houses supplied by the Lambeth Company, only 98 deaths occurred
• Cholera death rate was 8.5 times higher in houses supplied by the Southwark and Vauxhall Company
• Deaths from cholera are linked with the source of water

Epidemiologic Surveillance

• Snow needed data on cholera deaths collected by the British government to test his hypothesis
• Developed countries collect data on births, deaths, and other vital statistics often used for epidemiologic studies
• Governments use epidemiologic surveillance to recognize an epidemic is occurring before many people start dying
• Notifiable disease: A disease that the law requires to be reported to public health authorities as part of the public health surveillance system
• These conditions are infectious diseases whose spread can be prevented by the appropriate actions

Notifiable Diseases

• Physicians, hospitals, and clinical laboratories must report any case of a notifiable disease or condition to their local health department
• In turn, local health departments report to the state health department and the Centers for Disease Control and Prevention (CDC)
• Reporting cases of notifiable diseases allows public health authorities to detect an emerging epidemic at an early stage
• Control measures can then be enacted to control the spread of infectious diseases

Reporting of Chronic Diseases

• Reporting of chronic diseases is less widespread, but agencies have urged monitoring of conditions such as birth defects, Alzheimer’s, & asthma
• The monitoring system is designed to help identify environmental causes of chronic disease that could be controlled or eliminated
• In 2010, nearly all types of cancer were added to the list of national notifiable diseases
• Chronic disease: marked by long duration or frequent recurrence, incurable but not immediately fatal
• Common chronic diseases: cardiovascular disease, cancer, diabetes, Alzheimer's, and acquired immunodeficiency syndrome

Epidemiology and the Causes of Chronic Disease

• Before the mid-20th century, conditions such as heart disease and cancer were considered a natural part of aging
• No one tried to look for causes or tried to prevent them
• Cancer, heart disease, and other diseases of aging do not have single causes
• They develop over time and are often chronic, disabling more than rapidly fatal
• These diseases cannot be prevented or cured by any vaccine or “magic bullet.”
• Prevention requires understanding the causes of a disease and factors which influence its progression
• Protecting the public hinges on learning how to prevent or delay the onset of chronic diseases

Epidemiology and the Causes of Chronic Disease

• Most chronic diseases cannot be attributed to a single cause with the exception is of smoking influencing lung cancer
• Many different risk factors may play a part in causing a disease
• Risk factor: A characteristic that statistically increases a person's chance of developing a disease or being injured

Heart Diseases

• Deaths from heart disease increased during the first half of the 20th century
• After World War II, one in every five men was affected with heart disease before age 60

First Epidemiologic Study for Chronic Disease

• Framingham, Massachusetts was the site of the the first epidemiologic study of a chronic disease in 1948
• Over half of the middle-aged population (over 5000 people) was examined, and various data was recorded
• Data collected included weight, blood pressure, smoking habits, and the results of various blood tests
• Every two years, the same people were examined again for the rest of their lives

Study Results of Framingham Study

• After 10 years, the study identified high blood pressure, high blood cholesterol, and smoking as major risk factors for heart disease
• Concepts of “normal” blood pressure and cholesterol levels shifted
• Weight gain and lack of exercise were associated with increased blood pressure and cholesterol, and were associated with an increased risk of heart disease

Benefits of the Study

• By the 1970s, death rates from heart disease were declining in the United States
• The improvement was associated with reduction in risk factors in 1970, including lower blood cholesterol levels, lower blood pressure, and less smoking
• In 2015, the age-adjusted death rate from cardiovascular disease in the United States was 71% lower than it was in 1970
• A smoker’s risk of heart disease drops back to that of a nonsmoker soon after the smoker quits
• High-density lipoprotein (HDL) is the "good" kind of cholesterol that is protective
• Low-density lipoprotein (LDL) cholesterol is the “bad” kind
• Exercise elevates HDL cholesterol and protects against heart disease

Lung Cancer

• Epidemiologic studies seeking causes of cancer began soon after the Framingham Study
• Epidemiologists did not have many clues about possible causes or risk factors for most cancers
• An exception was the link between smoking and lung cancer

Lung Cancer

• Mortality from lung cancer began dramatically increasing after the 1930s
• The hypotheses proposed to explain the increase were tobacco smoking and air pollution
• Both tobacco smoking and air pollution had increased in the same period
• In early studies in the United States and England starting in the late 1940s, patients with lung cancer were questioned about their smoking habits
• A high proportion of these patients were heavy smokers

Other Epidemiologic Studies

• Two major epidemiologic studies started during late 1950 and early 1952 established a link between lung cancer and tobacco smoking
• British epidemiologists Richard Doll and A. Bradford Hill sent physicians in the United Kingdom a questionnaire
• The questionnaire asked if they were smokers, past smokers, or nonsmokers
• Smokers and ex-smokers were asked about their age at starting to smoke, amount of tobacco smoked, and when they had quit
• Over 40,000 doctors responded to the survey

Conclusions from the Study

• Lung cancer death rate was 20 times higher among smokers than nonsmokers
• The death rate increased as the amount smoked increased
• The death rate among ex-smokers was lower than among smokers, and declined as the time that had passed since the doctor quitting Smoking increased
• Contrast in lung death rates between smokers and nonsmokers was the same in rural and urban areas, showing pollution isn't a cause
• Deaths from heart attacks were significantly higher among heavy smokers aged 35 to 54 than among nonsmokers