PUBH 6003 ACTUAL FINALS ANSWERS AND
QUESTIONS SET A+
✔✔Prevalence Odds Ratio (POR) - ✔✔cross-sectional studies
•Odds of exposure among diseased compared to non-diseased (exposure-odds ratio)
•Odds of disease among exposed compared to unexposed (disease-odds ratio)
•Either yields the same OR, which is a prevalence odds ratio
•Calculated in the same manner as the odds ratio
This is commonly the outcome measure of choice in cross-sectional studies because its
statistical advantages outweigh difficulty in interpretation.
✔✔prevalence ratio (PR) - ✔✔Prevalence (exposed) / Prevalence (unexposed)
(a/a+b) / (c/c+d)
✔✔prevalence vs incidence in cross sectional studies - ✔✔•If results of cross-sectional
studies are interpreted as incidence (i.e. risk) instead of prevalence, that interpretation
will always be biased
--> Prevalence as an estimate of incidence is biased since P ~ IxD.
•If duration is long, prevalence may be greater than risk
•If duration is very short, prevalence may be less than risk
•Prevalent cases will overrepresent the cases with longer duration
•Longer lasting cases may be milder, and severity of disease may be associated with
exposures, so the disease exposure associating is also biased.
•Cross-sectional studies can produce unbiased estimates of prevalence, PR, and POR
only—but not of risk measures
, ✔✔infectious diseases - ✔✔•Any disease resulting from a biological agent reproducing
in the host
•Biological agents may be:
•Prions, viruses, bacteria, fungi, or parasites (i.e., protozoa or helminths)
Treatment (if any) and prevention methods depend on class of agents
Often acute, with short latency, but there are exceptions (see previous slide)
May or may not be communicable
✔✔communicable disease - ✔✔•Infectious disease which is transmissible between
humans either directly or indirectly.
•Contagious disease may be distinguished from communicable as being easily
transmissible directly from person-to-person contact.*
•Epidemiology began as the science of public health, which was focused almost
exclusively on combatting communicable disease.
✔✔Emerging infectious diseases (EIDs) - ✔✔•Category used to identify new or
reemerging infections according to Centers for Disease Control and Prevention (CDC)
•Inclusion on list helps to prioritize research needs
•Depends on surveillance to be identified as emerging
✔✔infectivity - ✔✔ability of agent to invade and multiply host
✔✔pathogenicity - ✔✔ability of agent to produce clinically apparent illness in host
✔✔virulence - ✔✔ability of agent to produce severe disease in a host
✔✔immuogenicity - ✔✔degree to which an agent provokes an immune response in host
✔✔incubation period - ✔✔time it takes from infection to clinical illness, generally given
as an average and the range from shortest to longest observed
✔✔iceburg concept - ✔✔•Not all diseases will have more undetected than detected.
•Less is missed with greater pathogenicity, virulence, or surveillance.
•More is missed with greater duration of latency or incubation period.
•Some diseases always result in clinically apparent disease in a short period of time
(like rabies or Ebola).
✔✔requirements for disease transmission - ✔✔•Presence of a susceptible host
•An infectious agent capable of causing an infection
•Presence of a reservoir
•Portal of exit from reservoir and entry into host
•Mode of transmission
✔✔Reservoir host - ✔✔•Reservoirs in infectious diseases are where the agent naturally
lives and reproduces.
QUESTIONS SET A+
✔✔Prevalence Odds Ratio (POR) - ✔✔cross-sectional studies
•Odds of exposure among diseased compared to non-diseased (exposure-odds ratio)
•Odds of disease among exposed compared to unexposed (disease-odds ratio)
•Either yields the same OR, which is a prevalence odds ratio
•Calculated in the same manner as the odds ratio
This is commonly the outcome measure of choice in cross-sectional studies because its
statistical advantages outweigh difficulty in interpretation.
✔✔prevalence ratio (PR) - ✔✔Prevalence (exposed) / Prevalence (unexposed)
(a/a+b) / (c/c+d)
✔✔prevalence vs incidence in cross sectional studies - ✔✔•If results of cross-sectional
studies are interpreted as incidence (i.e. risk) instead of prevalence, that interpretation
will always be biased
--> Prevalence as an estimate of incidence is biased since P ~ IxD.
•If duration is long, prevalence may be greater than risk
•If duration is very short, prevalence may be less than risk
•Prevalent cases will overrepresent the cases with longer duration
•Longer lasting cases may be milder, and severity of disease may be associated with
exposures, so the disease exposure associating is also biased.
•Cross-sectional studies can produce unbiased estimates of prevalence, PR, and POR
only—but not of risk measures
, ✔✔infectious diseases - ✔✔•Any disease resulting from a biological agent reproducing
in the host
•Biological agents may be:
•Prions, viruses, bacteria, fungi, or parasites (i.e., protozoa or helminths)
Treatment (if any) and prevention methods depend on class of agents
Often acute, with short latency, but there are exceptions (see previous slide)
May or may not be communicable
✔✔communicable disease - ✔✔•Infectious disease which is transmissible between
humans either directly or indirectly.
•Contagious disease may be distinguished from communicable as being easily
transmissible directly from person-to-person contact.*
•Epidemiology began as the science of public health, which was focused almost
exclusively on combatting communicable disease.
✔✔Emerging infectious diseases (EIDs) - ✔✔•Category used to identify new or
reemerging infections according to Centers for Disease Control and Prevention (CDC)
•Inclusion on list helps to prioritize research needs
•Depends on surveillance to be identified as emerging
✔✔infectivity - ✔✔ability of agent to invade and multiply host
✔✔pathogenicity - ✔✔ability of agent to produce clinically apparent illness in host
✔✔virulence - ✔✔ability of agent to produce severe disease in a host
✔✔immuogenicity - ✔✔degree to which an agent provokes an immune response in host
✔✔incubation period - ✔✔time it takes from infection to clinical illness, generally given
as an average and the range from shortest to longest observed
✔✔iceburg concept - ✔✔•Not all diseases will have more undetected than detected.
•Less is missed with greater pathogenicity, virulence, or surveillance.
•More is missed with greater duration of latency or incubation period.
•Some diseases always result in clinically apparent disease in a short period of time
(like rabies or Ebola).
✔✔requirements for disease transmission - ✔✔•Presence of a susceptible host
•An infectious agent capable of causing an infection
•Presence of a reservoir
•Portal of exit from reservoir and entry into host
•Mode of transmission
✔✔Reservoir host - ✔✔•Reservoirs in infectious diseases are where the agent naturally
lives and reproduces.
