Principles of Communicable Disease

Epidemiology

The study of disease in populations and of factors that determine its occurrence.

Distribution (1st Epi Triad)

The occurrence of health outcomes in populations, described by animal, place, and time.

Determinants

Any factor that brings about change in a health condition, including risk factors related to the host, agent, or environment.

Population Medicine

Epidemiology's focus on all organisms of the same group or species, contrasting with clinical medicine which targets individuals.

Infectious Disease

Illness due to a specific agent or its toxic products transmitted from a reservoir or infected host to a susceptible host either directly or indirectly.

Epidemic

The occurrence of an illness clearly in excess of normal expectancy in a community or region during a specific time.

Endemic

Disease that occurs at an expected frequency, is always present in a population, usually at a stable low level.

Pandemic

An epidemic occurring worldwide or over a very wide area, crossing international boundaries.

Sporadic Disease

Disease that occurs irregularly and haphazardly, producing small localized outbreaks.

Vertical Transmission

Infections transmitted from one generation to the next via infection of the embryo or fetus in utero or in ovo.

Horizontal Transmission

Infections transmitted from any segment of a population to another, either directly or indirectly.

Vector

An animate transmitter of infectious agents, usually invertebrate animals or arthropods like ticks.

Fomites

Inanimate carriers of infectious agents, such as transportation trucks, feed, or lab coats, surgical instruments, phones.

Reservoir

Any animal species, insect, soil, or substance in which an infectious agent normally lives, multiplies, and transmits from.

Second Epi Triad

The three interacting factor categories influencing disease risk: Host, Agent, and Environment.

Infectivity

The capacity of an agent to enter and multiply in a susceptible host and produce infection or disease.

Pathogenicity

The qualitative capacity of an agent to cause overt disease in the infected host.

Virulence

The quantitative capacity of an agent to induce a specific degree of pathology in the host.

Active Immunity

Immunity acquired by previous exposure to antigens of an agent via infection or vaccination.

Passive Immunity

Short-lived immunity acquired through transfer of antibodies, such as from colostrum or transplacental transfer.

Herd Immunity

Resistance of a group of animals to a disease due to a large percentage of the group being immune.

Latent Period

The time lapse between infection and the ability to transmit the infectious agent.

Incubation Period

The period of time between infection and the development of clinical signs.

Carrier

An individual who harbors an infectious organism subclinically without clinical signs, potentially shedding the agent.

Iceberg Concept of Disease

The model showing that observable clinical cases represent only the tip of an iceberg, while unobserved subclinical cases are more frequent.