Epidemiology Flashcards
science that evaluates occurrence, determinants,
distribution, and control of health and disease in a defined
human population
Epidemiology
occurs occasionally and at irregular intervals
example: histoplasmosis, gas gangrene
sporadic disease
maintains a relatively steady low-level frequency at a moderately regular interval
(example: gonorrhea, Dengue fever)
endemic disease
occurs at a high, constant level throughout a population
example: malaria
holoendemic diseases
– sudden increase in frequency above expected number
– index case – first case in an epidemic
(example: influenza, meningococcal infections)
epidemic
– sudden, unexpected occurrence of disease
– usually focal or in a limited segment of population
outbreak
– increase in disease occurrence within large population over wide region
(usually worldwide)
pandemic
• an incidence rate
Centers for Disease Control
• number of new cases in a specific time period per unit of population
(# new cases during a specific time/ # individuals in population) `
Morbidity rate
- total number of individuals infected at any one time per unit of population
- depends both on incidence rate and duration of illness
Prevalence rate
• number of deaths from a disease per number of cases of the disease
deaths due to given disease/ size of total population with disease
Mortality rate
______ = number of secondary cases one case would produce in a completely susceptible population
R0 or R(0), “R-naught”
What are the three parameters that used to calculate R(0)
• duration of contagiousness after a person becomes infected
• likelihood of infection per contact between a susceptible person
and an infectious person or vector • contact rate
• involves use of surveillance methods
• cases of a disease recognized by its characteristic
disease syndrome
– set of signs and symptoms characteristic of a disease
– signs
• objective changes in body that can be directly observed
– symptoms
• subjective changes experienced by patient
Recognition of an Infectious Disease in a Population
– period after pathogen entry but before signs and symptoms appear
• incubation period
– onset of signs and symptoms
– not clear enough for diagnosis
• prodromal stage
– disease is most severe and has characteristic signs and symptoms
• period of illness
– signs and symptoms begin to disappear
• convalescence
• after recognition of infectious disease in a population, outbreak
correlated with specific pathogen [e.g. HIV/AIDS (early 1980s), SARS
CoV (2003), SARS-CoV-2 (2019-2020)] • clinical microbiologists help in isolation and identification of pathogen
Correlation with a Single Causative Agent
What are the two types of epidemics?
Common-source epidemic
propagated epidemic
= minimum number of individuals necessary to continue propagating the disease
threshold density
• resistance of a population to infection and to spread of an
infectious organism because of the immunity of a large percentage of the population
• level can be altered by introduction of new susceptible
individuals into population
• level can be altered by changes in pathogen
– antigenic shift – major change in antigenic character of pathogen
– antigenic drift – smaller antigenic changes
Herd immunity
– can be transmitted from one
host to another
• communicable disease
– location from which pathogen is transmitted to host
source
– time during which source is
infectious or is disseminating
the organism
period of infectivity
– site or natural environmental location in which pathogen is normally found
– sometimes functions as source of pathogen
reservoir
– infected host
– can be casual (acute or transient)
carrier – short time – can be chronic carrier – long time
carrier
have the overt clinical case of disease
active carrier
– has recovered from disease but continues to harbor large numbers of pathogen
convalescent carrier
– harbors pathogen but is not ill
healthy carrier
– harbors pathogen but is not yet ill
incubatory carrier
organisms that spread disease from one host to another
vector
• pathogen suspended in air and travels >or = 1 meter
• droplet nuclei
– small particles (1-4 micrometers in diameter)
– can remain airborne for long time (hours to days)
– can travel long distances
– usually propelled from respiratory tract of source organisms by sneezing,
coughing, or vocalization
• dust particles also important route of airborne transmission
Airborne Transmission
• coming together or touching of source/reservoir and host
• direct contact (person-to-person)
– physical interaction between source/reservoir and host
– e.g., kissing, touching, and sexual contact
• droplet spread
– large particles (>5 micrometers) that travel < 1 meter
Contact Transmission
______= inanimate materials or objects involved in pathogen
transmission
Vehicles
- inanimate materials or objects involved in pathogen transmission
– water and food (spread pathogen to multiple hosts)
– fomites = inanimate objects: surgical instruments, bedding and eating utensils
vehicle transmission
– passive carriage of pathogen on body of vector
– no growth of pathogen during transmission
• external (mechanical) transmission
– carried within vector
– harborage transmission
– pathogen does not undergo changes within
vector (bubonic plaque: Yersinia pestis in flea) – biologic transmission – pathogen undergoes changes within vector
(malaria: Plasmodium spp. in Anopheles spp. mosquito)
internal transmission
What are the two main factors in determining why the host was susceptible to the pathogen?
– defense mechanisms of host
– pathogenicity of pathogen
– movement of pathogen to portal of exit [helminths (worms) that migrate through the body of the host]
active escape
– excretion in feces, urine, droplets, saliva, or
desquamated cells [microbes]
• passive escape
- Reduce or eliminate source or reservoir
- Break connection between source and susceptible individuals
- Reduce number of susceptible individuals
Control of Epidemics
ways to Reduce or eliminate source or reservoir
– quarantine and isolation of cases and carriers
– destruction of animal reservoir
– treatment of sewage
– therapy that reduces or eliminates infectivity of cases
Ways to Break connection between source and susceptible individuals
– chlorination of water supplies
– pasteurization of milk
– supervision & inspection of food & food handlers
– destruction of insect vectors with pesticides
ways to Reduce number of susceptible individuals
– passive immunity following exposure
– active immunity for protection
together these result in an augmented herd immunity
