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.

• 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:– 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– 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 non-exposed

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

– 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

• 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

– 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