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- epidemic limited to localised increase in the incidence of disease
- 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
- 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]\displaystyle{ 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 illness onset of primary case to illness onset of secondary case caused by primary case
- generation time
- time from infection of primary case to /u>infection of secondary case caused by primary case
SIR model
- S = susceptible
- I = infected
- R = recovered
- β = transmission coefficient (S→I)
- γ = recovery rate (I→R)
