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something physical, chemical, nutritional, psychosocial, or biologic that can cause a health issue; must be present for a disease to occur
airborne transmission
occurs when infectious agents are capable of remaining suspended in air over long distances and long periods of time, in contrast to droplets that fall to the ground within a few feet
the p value or the significance level for the results of a study to be deemed statistically significant
analytic epidemiology
the study of the causes and effects of diseases or other health events, looking for the why and the how, attempting to quantify a relationship between two variables
attack rate
the proportion of a population that develops an illness during an outbreak (synonym of incidence proportion)
attributable risk (AR)
a measurement of the amount of disease among exposed individuals that is attributed to the exposure
a systematic error that underestimates or overestimates the value of a parameter; a source of error in a study’s outcome arising in the design or conduct of the study, data collection, or data analysis
individuals with incubating disease or preclinical infection but without overt symptoms who are still capable of transmitting disease
case-control study
a study design in which investigators enroll a group of individuals with a disease and a group of individuals without the disease and compare previous exposures between the groups
case-fatality rate
the proportion of individuals with a disease who die from it
the relationship between cause and effect
chain of infection
an epidemiological model that allows for a complex and nuanced interplay between the host, agent, and environment of the epidemiological triad
a random error in a study
clinical disease stage
stage within the natural history of disease during which signs of the disease develop and diagnosis may occur
cohort study
a study in which the investigator documents whether or not study participants were exposed to what is being studied
confidence intervals
the relative risk (or other risk measure) found in the study and an interval within which the risk would most likely fall if the study were repeated multiple times
a type of error that may result in an incorrect causation or conclusion
cross-sectional study
a study in which a sample of individuals from a specified population is enrolled and the exposure and disease outcome are measured simultaneously for each participant; provides a snapshot of any specified population at a given point in time
death-to-case ratio
the number of deaths attributed to a disease during a specific period of time divided by the number of new cases of that disease during the same period
descriptive epidemiology
the frequency and pattern of health events within a population according to the characteristics of person, place, and time
direct transmission
transmission of an infectious agent from a reservoir to a host by direct contact or droplet spread of infected material
droplet spread
infected material transmitted by direct spray of relatively large, short-range aerosols of the pathogen over a few feet prior to the droplets falling to the ground
the continual and constant presence of a disease within a given geographic area
environmental factors
in the epidemiologic triad, the biologic environment of plants, animals, and toxins, including vectors that carry infectious agents and the reservoirs where infectious agents are normally found
when the level of disease in a defined area rises above endemic levels
epidemiological triad
a model of disease causation; classically describes disease as a result of the relationship between a susceptible host, an external agent, and an environment bringing the host and agent together
scientific study of the distribution and determinants of diseases and health outcomes in populations
experimental studies
in epidemiology, studies in which the investigator controls or changes the factors thought to cause a health event and then observes what happens to the health state
health events
disease, injury, or death
an individual, a family, a group of high-risk individuals, or a community within the context of the epidemiological triad
persistent high levels of disease in a defined area
incidence proportion
the proportion of a population that develops an illness during an outbreak (synonym of attack rate)
incidence rate
during a given time period, all new cases of a disease or health condition divided by the population at risk
indirect transmission
transmission that occurs when an infectious agent is transmitted from a reservoir to a host by suspended air particles, inanimate objects also known as vehicles, or vector intermediaries
having a disease, illness, or medical condition; includes disease, injury, and disability
mortality rate
the frequency of death in a defined population during a specific time interval
natural history of disease
events that occur before development of a disease, during the course of the disease, and at the conclusion of the disease
observational studies
epidemiological studies based on investigator observations of exposure and disease status
odds ratio (OR)
odds a case was exposed divided by the odds a control was exposed; quantitatively expresses the association between an exposure and a disease or health outcome; most often used to estimate the relative risk in case-control studies when the disease being investigated is rare
an epidemic affecting a limited geographic area
p value
the probability that an observed association could be the result of random error
the ability of an agent to cause disease, influencing disease onset
portal of entry
how an agent infects a susceptible host
portal of exit
how an agent leaves its home base in the chain of causation/infection
prevalence rate
proportion of a population that has a health condition at a certain point in time or over a time interval
a form of a ratio in which the numerator represents a subset of the denominator
rate ratio
compares the incidence rates or mortality rates of two groups
a comparison of any two values, calculated by dividing one interval by the other
relative risk (RR)
the ratio of the incidence proportion of the health event in exposed individuals (or the group of primary interest) to the incidence proportion in unexposed individuals (or the comparison group); compares the risk of a health event among one group with the risk among another group; see also risk ratio
where the causal agent normally lives and reproduces in the chain of causation
risk ratio
synonym of relative risk
sample size
the number of participants in a study
diseases that occur at irregular intervals
spot map
a type of epidemiological map that shows the geographic distribution of cases of illness or disease by marking each case with a dot on a map
statistical significance
an assessment for random error in a study
subclinical disease stage
the natural history of disease stage after exposure but prior to clinical disease in which individuals have pathologic change but no overt symptoms
susceptibility stage
the initial interactions between the agent, host, and environment during the pre-pathogenesis period, during which time primary prevention measures could be implemented to prevent disease onset

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