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Definition of epidemiology
The study of patterns (distribution and determinants) of health, disease, and injury in human populations and the application of this study to the control of health problems.
- Host (e.g. person)
- Agent (e.g. bacterium)
- Environment (e.g. contaminated water)
- *Vector (may also be involved, e.g. mosquito)
- *For such an interaction to take place, the host must be susceptible
The occurrence of disease in a community or region that is clearly in excess of normal expectations and derived from a common or propagated source.
The habitual presence of a disease within a given geographic area.
Pandemic (+ three conditions)
A worldwide epidemic.
A pandemic is declared based on the geographic distribution of the disease (not based on # of cases or severity).
- Three conditions:
- 1. A new infectious agent such that humans have no natural immunity.
- 2. Agent must evolve to be capable of infecting and killing humans efficiently.
- 3. Agent must succeed in being transferred from human to human.
4 underlying assumptions in epidemiology
- 1. Illness and disease are not randomly distributed in human populations
- 2. Each human being has characteristics that either predispose to illness, or protect from illness
- 3. Communities and neighborhoods also have characteristics that either predispose toward, or protect from illness
- 4. These characteristics are identifiable and modifiable
*5 Objectives of Epidemiology*
- 1. To identify the cause(s) of a disease and the risk factors for that disease.
- (How is the dz transmitted or acquired? Are there subgroups in the pop. at high risk for the dz?)
- 2. To determine the extent of the dz found in a community or population (surveillance).
- (What is the burden of the dz/condition?)
- 3. To study the natural hx and prognosis of the disease.
- (severity, lethality, duration, survivorship)
- 4. To evaluate new and existing preventive and therapeutic measures as well as modes of health care delivery.
- (Does screening for dz impact dz outcome?)
5. To provide the foundation for developing public health policy and regulating decisions.
The resistance of a group of people to an attack by a disease because a large proportion of the group members are immune (decreases the likelihood that any one person with the dz will come into contact with susceptible individuals).
- -the dz agent must be restricted to a single host species within which transmission occurs, and that transmission must be relatively direct from one member of the host species to another (i.e., there is not reservoir for the organism outside of the human host - if that were to happen then there would be other means of transmission available)
- -Infections/vaccinations must induce solid immunity (not partial immunity)
Difference in leading causes of death in 1900 vs. now
In 1900 the three leading causes of death were infectious disease (pneumonia/influenza, TB, diarrhea/enteritis)
In 2009 the leading causes of death were chronic diseases (heart disease, cancer, COPD, stroke).
Three types of prevention
- Primary prevention: Preventing the initial development of a disease in a person who is well and does not yet have the disease in question.
- (e.g. immunization, reducing risk of exposure to a risk factor)
- Secondary prevention: Early detection of existing disease in people in whom the dz process has begun, but who have not yet developed clinical signs/sx of illness - by catching the disease at this stage (often through screening) it is hoped that tx will be easier and/or more effective.
- (e.g. cancer screening)
- Tertiary prevention: Reducing the impact of the disease. Preventing complications in those who have already developed signs/sx of an illness and have been dx by providing prompt and appropriate tx.
- (e.g. rehabilitation for stroke)
1. Determine whether an association exits between exposure to a factor (e.g. an environmental agent) or a characteristic of a person (e.g. increased serum cholesterol) and the development of the disease in question.
2. If there is an association between an exposure and a disease, the second step is to try to derive appropriate inferences about a possible causal relationship from the patterns of association (not ALL associations are causal)
Modes of Disease Transmission
- 1. Direct
- a. Person-to-person contact
- 2. Indirect
- b. Common vehicle
- - Single exposure
- -Multiple exposures
- -Continuous exposures
- c. Vector
Clinical disease vs. Non-clinical disease
(including 4 types of nonclinical disease)
- Clinical disease:
- characterized by signs and symptoms
- Nonclinical disease:
- 1. Preclinical dz: disease that is not yet clinically apparent but is destined to progress to clinical disease
- 2. Subclinical dz: dz that is not clinically apparent and is not destined to become clinically apparent (often dx by serologic/antibody response or culture of organism)
- 3. Persistent (chronic) disease: A person fails to "shake off" the infection and it persists for years (sometimes for life)
- 4. Latent disease: an infection with no active multiplication of the agent (e.g. when a viral nucleic acid is incorporated into the nucleus of a cell as a provirus) (in contrast to persistent dz, only the genetic message is present in the host, not the viable organism)
An individual who harbors the organism but is not infected as measured by serologic studies (no evidence of antibody response) or by evidence of clinical illness. This person can still infect others, although the infectivity is often lower than with other infections.
The interval from the receipt of infection to the time of onset of clinical illness.
- Number of cases on y-axis; time on x-axis (or log of time to make it a normal shaped curve)
- Defined as the distribution of times of onset of the disease (in single-exposure, common-vehicle epidemics, the epidemic curve represents the distribution of incubation periods)
The three critical variables in investigating an outbreak or epidemic
- 1. When did the exposure take place?
- 2. When did the disease begin?
- 3. What was the incubation period of the disease?
(if we know any two of these, we can calculate the third)
number of people at risk in whom a certain illness develops ÷ total number of people at risk
-useful for comparing the risk of disease in groups with different exposures
Secondary attack rate
- The attack rate in susceptible people who have been exposed to a primary case (a primary case is a person who acquires the disease directly from an exposure like a contaminated food).
- -So this is the attack rate among secondary cases
- -It is a good measure of person-to-person spread of disease after the disease has been introduced into the population
- -also has application to noninfectious disease when family members are examined to determine the extent to which a disease clusters among first-degree relatives of an index case, which may yield clues regarding the relative contributions of genetic and environmental factors
Variables to consider when investigating the occurrence of a disease that appears to have occurred at more than an "endemic" level
- Who was attacked by the disease?
- When did the disease occur?
- Where did the cases arise?
Who: the characteristics of the human host are clearly related to disease risk (e.g. sex, age, race)
When: certain diseases occur with a certain periodicity, often there is a seasonal pattern to the temporal variation; the question of when also examines trends in increased dz incidence over time
Where: Dz is not randomly distributed in time or place.
Steps in investigating an acute outbreak
(The order of these steps may vary)
. Define the outbreak and validate the existence of an outbreak.
- a. define the "numerator" (cases)
- -clinical features - is the disease known?
- -what are its serologic or cultural aspects?
- -are the causes partially understood?
- b. define the "denominator" (population at risk, i.e. susceptible)
- c. determine whether the observed number of cases clearly exceeds the expected number
- d. calculate the attack rates
Examine the distribution of cases by the following:
- a. Time
- b. Place
- (Look for time-place interactions)
. Look for combinations (interactions) of relevant variables
. Develop hypotheses based on the following:
- a. existing knowledge (if any) of the dz
- b. analogy to diseases of known etiology
- c. findings from investigation of the outbreak
. Test hypotheses
- a. further analyze existing data (case-control studies)
- b. refine hypotheses and collect additional data that may be needed
. Recommend control measures
- a. control of current outbreak
- b. prevention of future similar outbreaks
. Prepare a written report of the investigation and the findings
. Communicate findings to those involved in policy development and implementation AND to the public