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==definition of outbreak by CDC==
*epidemic limited to localised increase in the incidence of disease
*[https://www.cdc.gov/csels/dsepd/ss1978/lesson1/section11.html Epidemic Disease Occurrence - Level of disease]
*Sporadic
**a disease that occurs infrequently and irregularly
*Endemic
**constant presence
**and/or usual prevalence of a disease
**or infectious agent in a population within a geographic area
*Hyperendemic
**persistent, high levels of disease occurrence.
*Epidemic
**increase, often sudden, in the number of cases of a disease above what is normally expected in that population in that area
*Outbreak
**the same definition of epidemic, but is often used for '''a more limited geographic area'''
*Cluster
**aggregation of cases grouped in place and time that are suspected to be greater than the number expected, even though the expected number may not be known
*Pandemic
**epidemic that has spread over several countries or continents, usually affecting a large number of people.
Epidemics occur when an agent and susceptible hosts are present in adequate numbers, and the agent can be effectively conveyed from a source to the susceptible hosts. More specifically, an epidemic may result from:
A recent increase in amount or virulence of the agent,
The recent introduction of the agent into a setting where it has not been before,
An enhanced mode of transmission so that more susceptible persons are exposed,
A change in the susceptibility of the host response to the agent, and/or
Factors that increase host exposure or involve introduction through new portals of entry.(47)
==what to do when to investigate an outbreak==
*[https://www.cdc.gov/csels/dsepd/ss1978/lesson6/section2.html Steps of an Outbreak Investigation]
# Prepare for field work
# Establish the existence of an outbreak
# Verify the diagnosis
# Construct a working case definition
# Find cases systematically and record information
# Perform descriptive epidemiology
# Develop hypotheses
# Evaluate hypotheses epidemiologically
# As necessary, reconsider, refine, and re-evaluate hypotheses
# Compare and reconcile with laboratory and/or environmental studies
# Implement control and prevention measures
# Initiate or maintain surveillance
# Communicate findings
==second attack rate (SAR)==
*proportion of individuals who are exposed to an infectious agent who become ill
<math>SAR=\frac{number_of_becoming_ill}{number_of_exposed}</math>
==reproduction number==
===basic reproduction number===
*average number of secondary cases caused by a single primary case in a fully susceptible population
===effective reproduction number===
*actual number of secondary cases per single primary case
==time course==
*incubation period
**followed by symptomatic p.
*symptomatic p.
*latent (pre-infectious) p.
**followed by infectious p.
*infectious p.
*serial interval
**time from <u>illness onset of primary case</u> to <u>illness onset of secondary case caused by primary case</u>
*generation time
**time from <u>infection of primary case</u> to /u>infection of secondary case caused by primary case</u>
==SIR model==
*S = susceptible
*I = infected
*R = recovered
*β = transmission coefficient (S→I)
*γ = recovery rate (I→R)
