dynamic of disease transmissionn part 1 Flashcards
what is a communicable disease ?
a disease that is caused by a transmission of a specific pathogenic agent to a susceptible host
what is the epidemiological triad ?
agent
host
environment
what are the components of the chain of infection ?
infectious agent leaves a host or a reservoir
through a portal of exit
and enters a susceptible host through a portal of entry
what are the factors that can describe an agent ?
infectivity infectiousness virulence pathogenicity infectious dose
how is infectivity measured ?
measured by the ratio of the number of people who become infected to the total number exposed to the agent
how is the infectiousness measures ?
by the basic reproduction number R0
and effective reproductive number
what is the basic reproduction number ?
it is the avergae number of secondary infections produced by a typical case of an infection in a population where everyone is susceptible
inn general for an epidemic to occur what is the R0 of a totally susceptible population ?
R0 must be more than 1 for an epidemic to occur in this case
what is the effective reproductive number ?
average number of secondary cases per infectious case in a population made up of both susceptible and non susceptible hosts
what conclusions can be drawn from different results and values of the effective reproductive number ?
R>1 the number of cases will increase
R=1 the disease is endemic
R<1 there will be a decline in cases
how can the effective reproductive number be estimated from the basic reproductive number ?
For example, if R0 for influenza is 12 in a population where half of the population is immune, the effective reproductive number for influenza is 12 x 0.5 = 6.
what value should the effective reproductive number reach inn order for herd immunity to be established ?
must fall below 1
what does the pathogenicity describe ?
the number of people who developed clinical symptoms in comparison to the total number of infected individuals
what does the virulence describe ?
is the ratio of the number of people with severe and fatal disease to the total number of people with disease
what are the type of human reservoirs ?
cases or carriers
what are the types of human cases in reservoirs ?
apparent and inapparent
what are the classifications of inapparent cases ?
preclinical
subclinical
latent
what are the preclinical cases ?
not clinically apparent but destined to progress to clinical
what are subclinical cases ?
not apparent clinically and are not destined to become apparent
how are subclinical cases identified ?
through serological testing or culture of the organism
what are the features of a latent infection case ?
no active multiplication of the organism , but only the genetic message is present in the host
what are the types of carriers ?
incubatory - shed during incubation period
contact/healthy
convalescent - shed w homa by5efo
what are the divisions of an infectivity period ?
transient - days
temporary - weeks up to 3 months
chronic - more than 3 months to life
what are zoonoses ?
single diseases that primarily infect and spread between animals but have the potential transmission to humans
what are the different modes of transmission ?
direct transmission
indirect transmission
airborne transmission
vertical transmission n
what is included under direct transmission ?
droplet transmission
direct contact
trans placental
what is included under indirect transmission ?
vehicle borne
vector borne
what is included under airborne transmission ?
droplet nuclei
dust
what is included under vertical transmission ?
in utero
perinatal
breast feeding
what is the incubation period ?
the period of time it takes for the 1st clinical sign to show after entry of the pathogen into the host
what is the period of communicability ?
period where the person is contagious
