In infectious disease epidemiology, an agent is an infectious organism or pathogen, such as a virus, bacterium, paraiste, etc. that can cause disease. 
Some factors that can influence an agent's ability to cause disease include: pathogenicity, virulence, addictive qualities, infectivity, ability to survive outside body, susceptibility to antibiotics, etc. 


Agent/Pathogen

Qualities that can affect an environment or its susceptibility include: climate, building design, sanitation, social environment, access to health care, population density, etc.


Environment

The traditional sequence in which infectious disease results from the interaction of agent, host, and environment.


Chain of Infection

Examples of factors that may make a host more susceptible to infection or disease are genetics, resiliency, nutritional status, prior exposure, age, co-infection, etc.


Host susceptibility

A vector is an organism which transmits infection by conveying the pathogen from one host to another without causing disease itself, may be part of the infectious process.

Vector

Traditional model for infectious disease that consits of an agent/pathogen, a susceptible host, and an environment.  Disease is a result of an agent/pathogen and a susceptible host interacting in an environment that enables transmission. 

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The study of the incidence, prevalence, and distribution of illness or disease in a given population

Epidemiology

Centers for Disease Control and Prevention. (n.d.). Centers for Disease control and Prevention. Centers for Disease Control and Prevention. https://www.cdc.gov/. 

Centers for Disease Control and Prevention

Regarding epidemiology, the expectation is that analyzing the distribution of cases in a community or region and uncovering the distinguishing characteristics of the affected individuals or groups will teach us something about the causes of a particular disease and the methods by which it spreads. Epidemiological methods can also be quite important in identifying groups of individuals who are at risk.

Epidemiological Methods 

Features of Epidemiology include:

- Etiology
- Disease transmission
- Morbidity
- Morbidity rate
- Mortality
- Mortality rate

Features of Epidemiology

Epidemiological Triangle

Pioneers in epidemiology include:
- John Snow
- Florence Nightingale
- Ignaz Semmelweis
- Joseph Lister
- Robert Koch
- Louis Pasteur

Pioneers in Epidemiology

Multiple international public health agencies have worked towards improving public health and healthcare programs, monitoring disease risks, and communicating and coordinating with other international agencies to uphold global public health.

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