• host
• infectious agent
• environment that promotes exposure of human host to the agent & determines whether an outbreak will occur

the study of the distribution (who, where, when) and determinants (risk factors) of infectious disease in populations

• host: age, sex, race, customs, occupation, genetic profile, immune status, nutritional status
• agent:bacteria, virus, fungi, parasites
• environment: temp, humidity, crowding, water treatment, milk pasteurization, food handling

what are examples of the various modes of transmission?

• direct
  • person to person (resp droplets-flu, contact-scabies)
• indirect
  • common vehicle (air-Tb, water-cryptosportidiosis, food-E.coli)
  • vector (insects-malaria/W Nile, animals-bats/rabies)

resistance of a group to an attack by a disease to which a large proportion of the members of the group are immune; once a certain percent of the population is immune, the likelihood is smaller that an infectious person will contact a non-immune person (dimished probability of exposure)

requires:

• single host species
• direct transmission
• full immunity

increase in the incidence of an event over the expected rates, or baseline rate that is usually found in that population (endemic)

what are some examples of rare infectious diseases, one case of which would be sufficient of an investigation to stop an outbreak?

• anthrax
• pneumonic plague
• rabies (human)
• smallpox
• ebola

• prevent further cases
• discover and describe "new" diseases (emerging pathogens)
• learn new mechanisms of transmission for "old" diseases
• evaluate existing prevention strategies
• address public concern about the outbreak

• time
• place
• person

whats the difference between a probable and a confirmed case?

probable case: no lab criteria available

confirmed case: clinical plus lab criteria met

the ability of a case definition to accurately identify those who have the new infection

(a)/ (a+c) x 100

a=true positives

c=false negatives (but actually positive)

the ability of the case definition to correctly identify those who do not have the new infection

d/(b+d) x 100

minimizing false positives (b) will lead to more accuracy in determining the cause of the outbreak

new cases

-----------------

population @ risk

*during same time period as the cases

individuals are at risk of disease if they:

• do not have disease at start of f/u period
• are capable of developing the disease
  • have organ of interest, ie
  • meet case definition and its restrictions on time, place, and person

what is plotted on an epidemic curve and what information can be determined by the shape of the curve?

• y axis: number of cases
• x axis: time
  • time period over which rate is calculated
  • at which each case presents
• shape of curve tells us:
  
  • common point source v. on going transmission
  • incubation period

how do single vs. multiple exposures show up on the epidemic curve?

single exposure: one peak, no index case

multiple exposure: bimodal (2 peaks)

continuous: no apparent peaks...level...

interval from receipt or transmission of the infection to the onset of clinical illness

during the incubation period, the person does not have any symptoms or signs of illness

disease specific

• source of infection
• mode of transmission

hypothesis developed with the help of:

• epidemic curve (pattern, incubation period)
• line listing (descriptive analyses of cases)

in a comparative study with controls (people at risk but w/o disease)