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Epidemiology
Epidemiology is:
• The study of the distribution and determinants of
health-related states or events in specified
populations and the application of this study to
the control of health problems.
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Using data to answer questions of:
Who is getting sick?
What is making them sick?
How can we use this information to reduce the
risk of others getting sick?
Disease surveillance
• The ongoing systematic collection, analysis, and
interpretation of outcome-specific data for use in planning,
implementation, and evaluation of public health practice.”
• Data collected in a surveillance system can be used for
many purposes, including:
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To estimate the magnitude of a health problem in a population
To understand the natural history of a disease
To detect outbreaks or epidemics
To document the distribution of a health event
To test hypotheses about causes of disease
To monitor changes in infectious organisms
Epidemiology is:
• The study of disease at a population level
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Endemic vs foreign disease
Sporadic vs epidemic vs pandemic
Disease outbreaks
Rate of disease
Incidence
Prevalence
Risk factors
Relative risk
Odds ratio
Relative risk
• The risk of an event (i.e., developing a disease)
relative to exposure
• A mathematical equation:
RR = probability of disease occurring in
exposed individuals/ probability in nonexposed
Odds ratio
• A measure of association: compares the odds of
disease in those exposed to the odds of disease
in those not exposed:
OR = odds of disease in exposed/odds of
disease in non-exposed
• An OR of 1 = no difference between groups, so
no association between hypothetical
exposure/cause and outcome = illness
Probability vs odds?
• The probability that an event will occur is the
fraction of times you expect to see that event
in many trials. Probabilities always range
between 0 and 1.
• The odds are defined as the probability that
the event will occur divided by the probability
that the event will not occur.
Association ≠ causation!!
• The causation fallacy
Reservoirs
• Human reservoirs: symptomatic vs
asymptomatic
• Non-human reservoirs
• Environmental reservoirs
Entry and exit strategies
• Portal or entry
• Portal of exit
• Disease transmission:
– Direct contact
• Fecal oral transmission
• Sexual transmission
– Indirect contact
• Fomites
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Droplet transmission
Air: droplet nuclei
Food
Vectors
Vectors
• Mechanical vectors
• Biological vectors
Epidemiology of disease
Pathogen
• Virulence factors
• Dose
• Incubation period
Host
• Susceptibility to a specific
pathogen: receptors
• Acquired immunity
– Active vs passive
– Natural vs vaccination
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Herd immunity
General health status
Age
Genetics
Culture
Types of epidemiologic studies
• Descriptive: who, what, where & when
• Analytical
– Cross sectional
– Retrospective
– Prospective
• Experimental
• Molecular
Cholera outbreak in Haiti 2010
• Identifying the source of the outbreak: a combination
of analytical and molecular epidemiology
Infectious disease surveillance
• National Disease Surveillance Network
• International Society for Infectious Diseases:
ProMED: http://www.promedmail.org/
• CDC: Morbidity and Mortality Weekly Report
(MMWR): http://www.cdc.gov/mmwr/
• Public health departments
• WHO
Emerging infectious diseases
• Novel diseases in a population OR diseases
that have a recently increased incidence
and/or distribution
• New vs newly recognized
Emerging infectious diseases
• Changes in microbes
– Microbial evolution: acquisition of virulence factors,
antimicrobial resistance, evasion, invasion or exit strategies
– Expansion of microbial and vector distribution
– Expansion of host range
• Environmental changes
• Changes in hosts
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Complacency
Global travel
Population expansion
New forms of crowding together in small spaces (i.e.,
daycare centers)
Cryptococcus neoformans var. gattii
• Increased incidence of cryptococcosis in healthy
individuals
• Spike in human cases preceded by a spike in canine
cases
Healthcare associated infections
• These come from:
– Other patients
– Healthcare environment
– Healthcare workers
– Patient’s own microbiota
Infectious disease transmission in a
health care setting
• Medical devices = fomites
• Healthcare personnel = direct transmission
• Airborne transmission