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human who harbors a pathogen and can transmit diseases to others without any sign or symptom of the diseases

communicable disease

disease where the pathogen is transmitted from a source through direct or indirect contact to a susceptible host resulting in infection

contagious disease

disease that easily and rapidly spreads from a source to a susceptible host through direct or indirect contact

droplet transmission

spread of microbes through droplet nuclei in air

epidemic disease

disease outbreak that occurs as a larger-than-expected number of cases occurring over a short time in a geographic region


science that investigates the cause, transmission, timing, and distribution of infectious disease episodes, with a focus on recognizing outbreaks, controlling those outbreaks, and treating the infected


study of the disease causes


inanimate surface or object that carries pathogenic microbes

herd immunity

situation in which a sufficient number of individuals in a population are immune to a contagious disease so that the disease spread is severely limited

holoendemic disease

disease having high prevalence in early childhood and infecting most of a population’s children, resulting in a total population in which children more often show evidence of the disease than adults

horizontal disease transmission

spread of pathogens between members of the same species not in a parent-offspring relationship

hyperendemic disease

disease that is present at a high and constant frequency in a population and equally affects all groups within that population


number of new disease cases recorded during a certain period of time

infectious disease

disease resulting from infection


characteristic of a disease that refers to how many pathogen particles are needed to infect a particular host

morbidity rate

frequency of a disease in a certain population

mortality rate

frequency of deaths in a population

nosocomial infection

health care–acquired infection

pandemic disease

disease outbreak affecting a large percentage of the world’s population over a vast (even worldwide) geographic region or regions

portal of entry

area of the body through which a pathogen can enter

portal of exit

area of the body through which a pathogen can be transmitted to others or to the environment


total number of cases within a population during a certain period of time

reservoir of infection

person, animal, plant, soil, or substance that harbors an infectious agent

sporadic disease

disease that occurs only occasionally or in scattered incidents


an organism that serves as a carrier of a pathogen between hosts with the pathogen having minimal effect on the fitness of the carrier

vehicle of transmission

object from which pathogens can be indirectly transferred to a person via touch or ingestion

vertical disease transmission

spread of pathogens from mother to offspring before or during birth

zoonosis (plural, zoonoses)

infectious disease spread via animal reservoirs, which can be both wild and domestic animals