Epidemiology

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attack rate

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term used in Infectious Disease Epidemiology to denote 
incidence proportion. It is a proportion measuring cumulative incidence for particular groups
observed for limited periods and under special circumstances, as in epidemic. It is frequently 
used to investigate infectious disease outbreaks (attack rate =  number of cases / number of 
people at risk)term used in Infectious Disease Epidemiology to denote 
incidence proportion. It is a proportion measuring cumulative incidence for particular groups
observed for limited periods and under special circumstances, as in epidemic. It is frequently 
used to investigate infectious disease outbreaks (attack rate =  number of cases / number of 
people at risk)

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Secondary attack rate

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the number of cases among familial or institutional contacts occurring within the accepted incubation period following exposure to a primary case

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Carrier

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a person or animal that harbors a specific infectious agent without discernible clinical disease and serves as a potential source of infection.

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Inapparent carrier

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individual with an infection that is inapparent throughout its course (also known as healthy or asymptomatic or subclinical carrier)

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Incubatory carrier

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exists during the incubation period of disease

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Convalescent and postconvalescent carriers

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individuals which had a clinically 

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Temporary or transient carrier

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individuals whose carrier state is short

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Chronic carrier

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individual in whom infectivity remains high for long periods after clinical illness has subsided

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Reservoir (of infectious agents)

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any animal, arthropod, plant, soil or substance (or combination of these) in which an infectious agent normally lives and multiplies, on which it depends primarily for survival, and where it reproduces itself in such a manner that it can be transmitted to a susceptible host.

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Infectious agent

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 entity that is capable of producing infection or infectious disease (for example, bacteria, viruses, parasites)

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Infection

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the entry and development or multiplication of an infectious agent in the body of persons or animals. Infection is not synonymous with infectious disease since the result of infection can be either inapparent infection or manifest infectious disease.

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Infectivity

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the ability of the infectious agent to enter, survive and multiply in the host.

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Epidemic

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the occurrence of cases of an illness with a frequency clearly in excess of

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common-­source epidemic

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stems from a single source of exposure to a causal 

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vector­-born epidemic 

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 epidemic due to transmission of infection through vectors (e.g., insects)

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person-­to-­person epidemic 

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sometimes called propagated epidemic, is one in which the causal agent is transmitted from one infected case to another

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Endemic

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usual prevalence of a given disease within such area

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Pandemic

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epidemic that occurs over a wide geographic area

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Epidemic curve

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graph of the distribution of cases of disease by time of onset. In a single exposure, common vehicle epidemic, the epidemic curve represents the distribution of incubation periods

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Incubation period

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the time interval between initial contact with infectious agent and the first appearance of symptoms

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Transmission of infectious agents

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any mechanism by which an infectious

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Direct mode of transmission

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direct transmission of infectious agent to the portal of entry (e.g., direct contact such as touching, biting, kissing or sexual intercourse; or by the droplet spread during sneezing, coughing or talking – usually limited to a distance of about 1 meter or less).

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Indirect mode of transmission

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indirect transmission of infectious agent through contaminated inanimate objects (e.g.,fomites – toys, soiled bedding) or contaminated food, water, or milk or any other substance or insect serving as an intermediate means by which an infectious agent is transported and introduced into a susceptible host 

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Routes of Transmission

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Person­ to­ person, air­borne, water­borne, food­borne, vector­borne, vehicle­borne.

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Vector

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Any agent (animal or microorganism) that carriers and transmits a disease. Transmission can be achieved either by a simple mechanical carriage (e.g., carriage by crawling or flying insect through soiling of its feet) or from an infected non­vertebrate host in whose organism the agent has propagated (multiplied).

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Pathogenicity

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the power of the infectious agent to cause apparent infection in an infected population (pathogenicity = number of cases / total number or infections (apparent + inapparent))

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Virulence

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the degree of pathogenicity of an infectious agent, indicated by case­fatality ratios (case­fatality ratio = number of deaths / number of cases)

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Susceptible

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a person or animal not possessing sufficient resistance against a particular pathogenic agent to prevent contracting infection or disease when exposed to the infectious agent

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Zoonosis

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diseases of animals of which man is only an incidental host

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Immunity

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resistance of the organism to the effects of particular infectious agent or its toxins. Effective immunity depends on the presence of antibodies or cells having a specific action on the infectious agents or its toxins.

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Active immunity

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attained either naturally by infection or by inoculation of the agent itself in killed, modified or variant form, or of fractions or products of the agent (e.g.,vaccination with the attenuated infectious agent)

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Passive immunity

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attained either naturally by transplacental transfer from the mother, or artificially by inoculation of specific protective antibodies

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Herd immunity

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the resistance of a group to invasion and spread of infectious agent, based on the resistance to infection of a high proportion of individual members of the group

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Control

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The reduction of disease incidence, prevalence, morbidity or mortality to a locally acceptable level as a result of deliberate efforts; continued intervention measures are required to maintain the reduction. Example: diarrhoeal diseases

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Elimination

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reduction to zero of the incidence of a specified disease in a defined geographical area as a result of deliberate efforts; continued intervention measures are required. Example: neonatal tetanus

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Eradication

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permanent reduction to zero of the worldwide incidence of infection caused  by a specific agent as a result of deliberate efforts; intervention measures are no longer needed.Example: smallpox.

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Quarantine

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restriction of the activities of well persons or animals who have been exposed to a case of infectious disease during its period of infectiousness to prevent disease transmission

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Isolation

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separation for the period of infectiousness of infected persons and animals from others to prevent or limit the direct or indirect transmission of infectious agents from those infected to those who are susceptible

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Primary level of prevention

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prevention of disease in people who do not yet have it (e.g., sex education programs to prevent HIV infection, vaccination campaigns, etc.)

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Secondary level of prevention

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early detection and intervention to reverse consequences (e.g., mammography to catch breast cancer at an early stage)

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Tertiary level of prevention

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intervention to minimize impairments and disabilities or to help patients adjust to irreversible conditions (e.g., palliative care in hospices)

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Epidemiologic triad

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host, environment and agent

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Surveillance

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ongoing systematic collection, analysis and interpretation of data essential to planning, implementing and evaluating public health practices, and dissemination of this information to those who need to know

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Passive surveillance

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Data supplied (e.g., by doctors) 

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Active surveillance

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Data actively collected (e.g, during outbreaks)

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Sentinel surveillance

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“Early warning system”

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Steps in an Outbreak Investigation

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1. Define the epidemic. 2. Describe the distribution of cases in terms of person, place and time. 3. Look for combinations (interactions) of relevant variables. 4. Develop hypotheses based on the following: 5. Test hypotheses. 6. Recommend control measures

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Reproductive rate (also basic reproduction number (R0))

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the average number of new infections that one infectious case generates during his/her infectious lifetime in a community of susceptible individuals. R0 can be reduced either through behavioral, educational or medical interventions. If R0 is reduced to below 1, then epidemic eradication occurs, because each infected individual (on average) will generate less than one new infection. The greater the value