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Epidemiology
(3 categories of study)
– The study of the occurrence, distribution, and determinants of health and disease in a population
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In developed countries infectious diseases cause ___ deaths than noninfectious diseases
fewer
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Worldwide, infectious disease accounts for __% of all deaths
30%
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Infectious disease hit an all time low around 19__
1980
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Chronic infections:
- -host and pathogen survive
- -New pathogens sometimes emerge for which the host has no resistance
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Acute infections:
pathogen eithers kills or is killed not chronic
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Epidemiologists trace the spread of disease to identify its ___ and mode of ___
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A disease is an ___ when it occurs in a large number of people in a population at the same time
epidemic
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A ___ is widespread, usually worldwide
pandemic
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An ___ disease is constantly present in a population, usually at low incidences
endemic
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The ___ of a disease is the number of newcases of the disease in a given period of time
incidence
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The ___ of a disease is the total number of new and existing cases in a population in a given time
prevalence
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arrange diseases according to prevalance
epidemic
pandemic
endemic
endemic < epidemic < pandemic
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subclinical infections
- -Diseased individuals who show no or mild symptoms
- -Subclinical individuals are called carriers
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Mortality
is the incidence of death in a population
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Morbidity
-incidence of disease including fatal and nonfatal diseases (all)
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Infection:
the organism invades and colonizes the host
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Incubation period:
the time between infection and onset of symptoms
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Acute period:
the disease is at its height
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Decline period:
disease symptoms are subsiding
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Convalescent period:
patient regains strength and returns to normal
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Reservoirs
- -sites in which infectious agents remain viable and from which infection of individuals can occur
- –humans, animals and nonliving matter can all serve as reservoirs
- • For example, soil is a reservoir for Clostridiumtetani, the cause of tetanus
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Zoonosis
- -any disease that primarily infects animals, but is occasionally transmitted to humans
- – Control of a zoonotic disease in the human population may not eliminate the disease as a potential public health problem
- – Certain infectious diseases have complex lifecycles involving an obligate transfer from a nonhuman host to humans followed by transfer back to the nonhuman host
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Carriers
- – Pathogen-infected individuals showing no signsof clinical disease, i.e. Typhoid Mary
- – Potential sources of infections
- – May be individuals in the incubation period of the disease
- – Can be identified using diagnostic techniques, including culture and immunoassays
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Epidemiologists follow transmission of a disease by correlating:
(4 types of data)
- geographic
- climatic
- social
- and demographic data
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Direct host-to-host transmission
– Infected individual transmits a disease directly to a susceptible host without the assistance of an intermediary (e.g., flu, common cold, STDs,ringworm)
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Indirect host-to-host transmission
– Occurs when transmission is facilitated by a living or nonliving agent
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vectors
Living transmission agents
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fomites
Nonliving transmission agents
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A common-source epidemic
(example)
- usually arises from contamination of water or food
- • Example: cholera
- -spike quickly and die off quickly
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host-to-host epidemic
(characteristic)
(example)
- the disease shows a slow, progressive rise and a gradual decline
- • Example: influenza and chicken pox
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why is coevolution of a host and its parasite common?
- Virulence of the parasite in host-to-host transmission diminishes and resistance of the host increases
- (e.g., myxoma virus introduced to control rabbits in Australia)
- • A host-to-host pathogen that kills its host before it can infect another host may become extinct
- -as the grow together virulence and mortality should drop to optimum levels
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Herd immunity
-the resistance of a group to infection due to immunity of a high proportion of the group
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How does Herd immunity work?
- – If a high proportion of individuals are immune to an infection then the whole population will be protected
- – Immunized people protect nonimmunized people because the pathogen cannot be passed on and the cycle of infectivity is broken
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