What is public health?
- "Organized community efforts aimed at the
prevention of disease & promotion of health"(Institute of Medicine, Future of Public Health)
- "The science and art of protecting and improving the health of communities through education, promotion of healthy lifestyles, & research for disease & injury prevention" (ASPH)
What is epidemiology?
"The study of the distribution and
determinants of health-related states and
events in populations, and the application of
this study to control health problems"
- refers to measurement of disease frequency,
involving quantification of the existence or
occurrence of disease.
- Examples: surveillance, observation, hypothesis testing, descriptive research, analytic research, experimental research
- frequency of disease by person, place
- Look for variations from a uniform distribution, i.e., does disease cluster?
- any factor or event that brings about
a change in a health condition
E.g. physical, biological, social, cultural,
environmental and behavioral factors that influence health
Health-related states and events
- infectious diseases, chronic diseases, deaths, injuries, disabilities, mental disorders, suicide, substance abuse, behaviors, use of health services, adverse events
- Used broadly to refer to 'determinants'
- agents, interventions, conditions, policies or anything that might affect health
- commonly used to refer to all health-related states or events
- refers to illness
- refers to death
- Epidemiology examines disease occurrence among population groups rather than among individuals.
- Sometimes referred to as "population health"
Application of this study to control health problems
- explicitly states the aim of epidemiology, which is to promote, protect and restore health
What is epidemiology, really?
The study of health and disease in populations
The basic science of public health
-Who gets disease?
-What causes disease?
-How does disease spread?
-What prevents disease?
-What works in controlling disease?
Scope of Epidemiology
• To describe the health status of populations
• To explain the etiology of disease
• To predict the occurrence of disease
• To control the distribution of disease
• Derived from term 'epidemic'
• Epi (upon)+ demos (people)+ logy (study of)
• Epidemic = unusual or increased frequency of an adverse effect in the population; in excess of
"normal" or "usual" expectancy
Why consider history of epidemiology?
- Scientific understanding of disease & causal factors is a product of historical successes & failures
- Changes in science, technology & disease patterns influence evolution of public health & epidemiology
- Address changing needs, science of epidemiology continues to evolve, developing new methodologies to deal w/ complex problems
Hippocrates (460-377 BC)
- Recognized that disease results from physical
environment rather than supernatural causes
o "On Airs, Waters and Places" (400 BC)
o Associated disease with place (geography), H2O conditions,climate, eating habits & housing
- Used terms "epidemic" and "endemic"
- Believed disease was result of imbalance in 4
bodily humors - black & yellow bile, phlegm, blood
James Lind (1753)
•Observed the effect of time, place,
weather and diet on the spread of scurvy
•Subsequent experiments with sailors
Edward Jenner (1796)
- Observation on the nature of cowpox
- Subsequent development of smallpox vaccine
Ignaz Semmelweis (1840s)
- Childbed Fever (puerperal fever)
- Developed hypothesis regarding "cause" & tested an intervention (hand-washing)
John Snow (1840)
- Father of Epidemiology
- Source of Cholera
- His maps of cholera cases
Joseph Goldberger (1914)
- non infectious origin of pellagra
- careful systematic observations of phenomena
Examples of Current Public Health Issues
• HIV/AIDS - especially internationally
• "Epidemic" of obesity - especially in children
• Health effects of environmental exposures
• Use of biological weapons - approaches
to detection? Small-pox vaccination?
• Causes of many important diseases remain unexplained
Two Fundamental Assumptions
• Human disease does not occur at random
• Human disease has causal & preventive factors that can be identified through systematic
• Essence of epidemiology= Comparison
The Epidemiologic Triangle
- for disease to occur there must be an interaction between: host, agent, & environment
The Epidemiologic Triangle
• Host factors
- Biologic traits
- Social traits
- Biologic (bacteria, viruses)
- Chemical (alcohol, environmental toxins)
- Physical (trauma, radiation, fire)
- Nutritional (excess or deprivation)
The Epidemiologic Triangle
o Physical (climate, urban/rural, pollution, altitude)
o Social factors (political, social, economic)
o Herd immunity
3 Major Contributors
1) Basic sciences - (e.g., biochemistry, physiology, pharmacology, pathology)
2) Clinical sciences- (e.g., clinical medicine, neonatology, obstetrics & gynecology, neurology, urology, oncology)
3) Population health - (also known as community medicine, preventive medicine, social medicine, public health or epidemiology)
o Examines disease occurrence among individuals
o Describes specific signs & symptoms, e.g. high fever, headache, malaise, vomiting, diarrhea
o Prescribes individual treatment
o Examines disease occurrence among populations
o Describes age groups affected, time trends,
geographic trends, other variables that affect
distribution of disease
o Prescribes interventions for the community & evaluates their effectiveness
• Epidemiology is dependent on clinical
medicine for accurate diagnoses
• Clinical observations may suggest need for
further epidemiologic investigation
o e.g. similar complaints from other patients
Complementary Approaches (cont'd)
• Epidemiologic knowledge helps guide
selection of diagnostic procedures
o What tests are the most valid?
o What is the most likely diagnosis given what is known about the patient & what is known about the distribution of disease?
• adds to understanding of the biologic basis of disease causation & prevention
o (e.g. how cholesterol contributes to plaque formation & heart attack) & "biologic plausibility"
• Laboratory studies provide precise control of
covariates - genetics, environment, measurement
Public health relevance of lab findings uncertain
o Differences between in vitro & in vivo systems
o Differences in susceptibility across species
o Difficulty extrapolating across dosages, routes of administration, lifespan
o Problems generalizing results from highly controlled setting to free living populations
o can quantify the magnitude of the association in humans
o can contribute to public health interventions long before basic biological
mechanisms of disease are understood
• e.g., smoking and lung cancer, toxic shock syndrome, thalidomide
Limitations of Experimental Design for Studies of Disease Causation
How to experiment in human populations?
• Ethical concerns
• How to meet the challenge?
o Development of specialized research strategies - observational studies
o e.g., case-control, cohort studies
Research Strategies (Study Designs)
Used In Epidemiology
• Experimental - Clinical Trials & Field Trials
• Observational (Non-Experimental): DESCRIPTIVE - patterns of disease by person, place & time &
ANALYTIC- testing hypotheses
• Cohort (prospective and retrospective)
• Suspect the possible influence of a factor on
the occurrence of disease
• Suspicion arises through observations
• Leads to formulation of specific hypothesis
• Test hypothesis in study using appropriate
• Systematically collect and analyze data to
determine whether statistical association exists
• Assess validity of results - chance, bias,
• Assess causality
Applications of Epidemiology
• To provide the scientific basis to prevent disease and injury and promote health:
o By conducting surveillance of disease and injury occurrence in populations
o By studying past and future trends in health and illness
o By identifying sections of the population at greatest risk to target intervention
Applications of Epidemiology
o By evaluating the effectiveness of programs in
improving the health of the population
o By studying the natural history of disease
o By identifying the CAUSES of disease
o By investigating disease outbreaks
o Diseases don't occur by chance
o Causes can be identified and predicted through systematic study
Essence of Epidemiology
Uses of epidemiology
o Identification of causes of disease
o Monitoring the frequency & distribution of disease
o Measuring the disease burden in populations
o Evaluating the impact of intervention programs
Sub-specialty areas of application
o Infectious diseases
o Chronic diseases
o Social epidemiology,
o Health services research, . . . others