Overview of Epidemiology
Epidemiology is the study of health and disease in human populations. It looks at the distribution
and determinants of health states (diseases and conditions e.g diabetes, asthma, epilepsy) and
health events (e.g. an observed health outcome e.g abnormal blood sugar test, seizure, SOB
attack). It also deals with how study findings can be used to improve population health.
Common states and events are related to death and diseases. We mostly look at bad outcomes
but also look at good outcomes as well
Disease- A condition where there is impaired normal function. Has signs and symptoms. The
issue with most diseases is that they have overlapping signs and symptoms. This is why
epideiology uses rigid definitions (case definitions) on what characterizes a disease. These
definitions become more specific as more information is gathered
At the beginning of an outbreak, the definition should be very broad so you don’t miss new
cases but the broadness must be within reason. Even with concrete testing methods for a
disease the vague definition because not everyone has access to the testing services
Case Classification
- Probable cases: meets clinical criteria and presumptive lab tests
- Confirmed cases: confirmatory lab test
What is a presumptive lab test??
Changing the definition of lab diagnostics can lead to a significant change in the number of
cases suddenly especially if we lower the threshold for a person having a disease in lab tests
Asking the person is not really the most accurate way to determining whether a disease is
present in a person. They might not know their diagnosis, they will need formal diagnosis from
dr, they might be a hypochondriac
National Death Index is a way of determining death rates in National Death Index
We have to look at all the data the same way. E.g You look at death certificates only not
choosing some people to look at death certificate information and toxicology reports for some
Death certificates can be very vague. Many have cardiac arrest even though cardiac arrest is
seen in every death. There needs to a core cause of death.
Epidemiology can be divided into:
- Descriptive epidemiology: Patterns are presented at the surface level showing disease
distribution among persons, place, and time. These research projects generate
hypotheses for people figuring out why the these trends are seen
- Analytic epidemiology: Identifying the reasons for the diseases trends. Hypothesis
testing
Features of descriptive epidemiology:
Disease distribution asks all the wh- questions of disease distribution
a. How common is the disease? Diseases are endemic (usual baseline level), epidemic,
and pandemic (epidemic that is widespread across the country, continent, or even the
world)
Rare diseases: Less than 200,000 in US have according to the rare disease act
b. Who gets disease? Diseases are not usually random. Some people are more likely to be
exposed and develop the disease. Talking about exposure and susceptibility of disease.
We usually look at demographics: age, race (collected not for genetics but for racism
, factors and lived experiences, natal sex/gender identity, marital status, education,
income, occupation
c. Where does the disease occur?
d. When does the disease occur?
Socioeconomic status- Education, income, occupation (factors related to how much money you
make)
Sometimes age groups might be grouped together but not equivalent to one another (e.g 5-14,
25-44, and 45-64 etc). This is done on purpose for the sake of grouping based on certain
characteristics (e.g based on school age, independence, sexual activity, stages in life, age risk
guidelines from bodies like CDC, strength of immune system)
Ratios are better than absolute numbers because it accounts for the total number of people in
each group being studied.
(Birth) Cohort effect- People of different ages belong to people of different ages because they
lived through different experiences and had unique exposures
There are other types of cohorts like occupational cohorts (poultry workers and COVID-19,
nurses and mental health issues)
MMWR- The CDC’s morbidity and mortality weekly report
Kaposi Sarcoma, a type of cancer, increases with HIV infection
Types of Descriptive Study Designs (Why is longitudinal not part??)
a. Case report and case series
i. Case reports used to be common papers but not anymore but HIPAA has
ensured that peoples privacy would not be violated. The case reports often
describe the person enough to be identifiable.
ii. Case series(usually 2-20 people) are a few case reports that present with very
similar key characteristics
b. Correlational Studies: Finding relationship between a factor and a disease in a
population of intrest. Note that correlation is not the same thing as causation. As an
epidemiologist, you need to have skepticism about whether a factor/exposure just
correlates with or causes a disease. These studies are not able to eliminate confounders
as possible explanations for the results. There is the risk of ecologic fallacy where the
relationship between exposure and disease made from the population is unjustly applied
to individual cases when it might not be true from one individual to the next
c. Cross sectional studies: exposures and diseases are assessed simultaneously among
individuals in a well-degined population. These studies have the ability to gather a lot of
data.
i. E.g the Health Interview Survey which is a national cross sectional survey that is
extensive. It is a questionnaire that asks individuals about their personal,
demographic, health information as of the date the survey is filled
Primary prevention- We are directly working against an outcome e.g disease and accidents
a. Passive (no individual effort has to be made as the environment and systems are set up
to support health)- Fluoridation, Air bag, folic-acid fortified bread
, b. Active (individuals have to take specific action for their health)- Fluoride mouth wash,
Seat belt, and use of vitamin supplement
Secondary prevention- Not necessarily preventing disease occurence but prioritizing the early
detection to improve disease outcome. E.g PKU screening, pap smear, mammography
Tertiary prevention- Disease present, but treatment, control, and management of disease is
looked at e.g mobility aids for the disabled.
Store bought bread has folic acid as a way to prevent neural tube defect in fetuses
Prior to getting pregnant, they will give you medication like folic acid included
Ecologic fallacy- Risk factor and disease are associated at the population level, but not at the
individual so the risk factor might not increase risk of disease for each individual
Exposed? Yes= E+
Exposed? No= E-
Risk Ratio Calculation (rate of exposure/rate of non exposure). Risk ratios higher than 1 are
increased risk and lower than 1 is decreased risk. Risk ratios greater than 2.5 are considered to
be HUGE!
The research process is a cycle: research question- study design- data collection- data
analysis- study results- interpretation and conclusions- research question
Errors are possible at every single point in the research process
History of Epidemiology: Selected Milestones
Most of the history is represented by certain sexes and racial groups
Hippocrates in 400BC: Looked at infectious disease. Began surveillance. Published first case
series reports. Discounted the idea that diseases were caused by supernatural forces.
Much of the historical epidemiology is infectious disease
The plague began the vital statistic tracking in london
John Gaunt in the 1600s (Descriptive epidemiology): First known person to examine time
trends, specifically mortality trends. First known Life Table (which summarizes probability of
survival and mortality for different age groups in a populations)
William Farr- First compiler of vital statistics: Birth, Death, Marriage. Comparred deaths by
cause and characteristics
18th century has major scientific advancements in the form of early vaccines (e.g for smallpox)
Small pox was 1st described in ancient chinese and sanskrit scripts. 1st evidence of disease is
10,000 BC
Variolation- 1st form of vaccinations. Materials from the scab of persons with mild smallpox
were blown into the nose of uninfected individuals. Widely practiced in Africa, India, and China.
It was a very dangerous procedure for both people administering and receiving the vaccine
Onesimus was brought to the US in early 1700 as a slave from ghana and introduced the idea
of variolation
Lady Montagu variolated her own son and made variolation popular among the british political
elite
Vaccine hesitancy has been around basically since vaccines became a thing
Vaccination has led to smallpox eradication. Smallpox is the only globally eradicated disease.
River blindness is getting that status soon.
John Snow: Father of infectious disease epi-Physician, surgeon, anaesthesiologist in London.
Epidemiology is the study of health and disease in human populations. It looks at the distribution
and determinants of health states (diseases and conditions e.g diabetes, asthma, epilepsy) and
health events (e.g. an observed health outcome e.g abnormal blood sugar test, seizure, SOB
attack). It also deals with how study findings can be used to improve population health.
Common states and events are related to death and diseases. We mostly look at bad outcomes
but also look at good outcomes as well
Disease- A condition where there is impaired normal function. Has signs and symptoms. The
issue with most diseases is that they have overlapping signs and symptoms. This is why
epideiology uses rigid definitions (case definitions) on what characterizes a disease. These
definitions become more specific as more information is gathered
At the beginning of an outbreak, the definition should be very broad so you don’t miss new
cases but the broadness must be within reason. Even with concrete testing methods for a
disease the vague definition because not everyone has access to the testing services
Case Classification
- Probable cases: meets clinical criteria and presumptive lab tests
- Confirmed cases: confirmatory lab test
What is a presumptive lab test??
Changing the definition of lab diagnostics can lead to a significant change in the number of
cases suddenly especially if we lower the threshold for a person having a disease in lab tests
Asking the person is not really the most accurate way to determining whether a disease is
present in a person. They might not know their diagnosis, they will need formal diagnosis from
dr, they might be a hypochondriac
National Death Index is a way of determining death rates in National Death Index
We have to look at all the data the same way. E.g You look at death certificates only not
choosing some people to look at death certificate information and toxicology reports for some
Death certificates can be very vague. Many have cardiac arrest even though cardiac arrest is
seen in every death. There needs to a core cause of death.
Epidemiology can be divided into:
- Descriptive epidemiology: Patterns are presented at the surface level showing disease
distribution among persons, place, and time. These research projects generate
hypotheses for people figuring out why the these trends are seen
- Analytic epidemiology: Identifying the reasons for the diseases trends. Hypothesis
testing
Features of descriptive epidemiology:
Disease distribution asks all the wh- questions of disease distribution
a. How common is the disease? Diseases are endemic (usual baseline level), epidemic,
and pandemic (epidemic that is widespread across the country, continent, or even the
world)
Rare diseases: Less than 200,000 in US have according to the rare disease act
b. Who gets disease? Diseases are not usually random. Some people are more likely to be
exposed and develop the disease. Talking about exposure and susceptibility of disease.
We usually look at demographics: age, race (collected not for genetics but for racism
, factors and lived experiences, natal sex/gender identity, marital status, education,
income, occupation
c. Where does the disease occur?
d. When does the disease occur?
Socioeconomic status- Education, income, occupation (factors related to how much money you
make)
Sometimes age groups might be grouped together but not equivalent to one another (e.g 5-14,
25-44, and 45-64 etc). This is done on purpose for the sake of grouping based on certain
characteristics (e.g based on school age, independence, sexual activity, stages in life, age risk
guidelines from bodies like CDC, strength of immune system)
Ratios are better than absolute numbers because it accounts for the total number of people in
each group being studied.
(Birth) Cohort effect- People of different ages belong to people of different ages because they
lived through different experiences and had unique exposures
There are other types of cohorts like occupational cohorts (poultry workers and COVID-19,
nurses and mental health issues)
MMWR- The CDC’s morbidity and mortality weekly report
Kaposi Sarcoma, a type of cancer, increases with HIV infection
Types of Descriptive Study Designs (Why is longitudinal not part??)
a. Case report and case series
i. Case reports used to be common papers but not anymore but HIPAA has
ensured that peoples privacy would not be violated. The case reports often
describe the person enough to be identifiable.
ii. Case series(usually 2-20 people) are a few case reports that present with very
similar key characteristics
b. Correlational Studies: Finding relationship between a factor and a disease in a
population of intrest. Note that correlation is not the same thing as causation. As an
epidemiologist, you need to have skepticism about whether a factor/exposure just
correlates with or causes a disease. These studies are not able to eliminate confounders
as possible explanations for the results. There is the risk of ecologic fallacy where the
relationship between exposure and disease made from the population is unjustly applied
to individual cases when it might not be true from one individual to the next
c. Cross sectional studies: exposures and diseases are assessed simultaneously among
individuals in a well-degined population. These studies have the ability to gather a lot of
data.
i. E.g the Health Interview Survey which is a national cross sectional survey that is
extensive. It is a questionnaire that asks individuals about their personal,
demographic, health information as of the date the survey is filled
Primary prevention- We are directly working against an outcome e.g disease and accidents
a. Passive (no individual effort has to be made as the environment and systems are set up
to support health)- Fluoridation, Air bag, folic-acid fortified bread
, b. Active (individuals have to take specific action for their health)- Fluoride mouth wash,
Seat belt, and use of vitamin supplement
Secondary prevention- Not necessarily preventing disease occurence but prioritizing the early
detection to improve disease outcome. E.g PKU screening, pap smear, mammography
Tertiary prevention- Disease present, but treatment, control, and management of disease is
looked at e.g mobility aids for the disabled.
Store bought bread has folic acid as a way to prevent neural tube defect in fetuses
Prior to getting pregnant, they will give you medication like folic acid included
Ecologic fallacy- Risk factor and disease are associated at the population level, but not at the
individual so the risk factor might not increase risk of disease for each individual
Exposed? Yes= E+
Exposed? No= E-
Risk Ratio Calculation (rate of exposure/rate of non exposure). Risk ratios higher than 1 are
increased risk and lower than 1 is decreased risk. Risk ratios greater than 2.5 are considered to
be HUGE!
The research process is a cycle: research question- study design- data collection- data
analysis- study results- interpretation and conclusions- research question
Errors are possible at every single point in the research process
History of Epidemiology: Selected Milestones
Most of the history is represented by certain sexes and racial groups
Hippocrates in 400BC: Looked at infectious disease. Began surveillance. Published first case
series reports. Discounted the idea that diseases were caused by supernatural forces.
Much of the historical epidemiology is infectious disease
The plague began the vital statistic tracking in london
John Gaunt in the 1600s (Descriptive epidemiology): First known person to examine time
trends, specifically mortality trends. First known Life Table (which summarizes probability of
survival and mortality for different age groups in a populations)
William Farr- First compiler of vital statistics: Birth, Death, Marriage. Comparred deaths by
cause and characteristics
18th century has major scientific advancements in the form of early vaccines (e.g for smallpox)
Small pox was 1st described in ancient chinese and sanskrit scripts. 1st evidence of disease is
10,000 BC
Variolation- 1st form of vaccinations. Materials from the scab of persons with mild smallpox
were blown into the nose of uninfected individuals. Widely practiced in Africa, India, and China.
It was a very dangerous procedure for both people administering and receiving the vaccine
Onesimus was brought to the US in early 1700 as a slave from ghana and introduced the idea
of variolation
Lady Montagu variolated her own son and made variolation popular among the british political
elite
Vaccine hesitancy has been around basically since vaccines became a thing
Vaccination has led to smallpox eradication. Smallpox is the only globally eradicated disease.
River blindness is getting that status soon.
John Snow: Father of infectious disease epi-Physician, surgeon, anaesthesiologist in London.
