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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. • • • • 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 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 – – – – 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