Introduction to Epidemiology

• Epidemiology studies the distribution and determinants of health-related states or events in specified populations to control health problems
• The goal is to limit undesirable health events in a community

Principles of Epidemiology

• Epidemiology uses quantitative data and scientific study designs to examine relationships between exposure and diseases
• Epidemiologists search for patterns and frequency of diseases in populations
• A goal of epidemiology is to identify risk factors, contributing agents, exposures, or causes of diseases
• Health-related states extend to chronic illnesses, genetic diseases, environmental health issues, mental conditions, occupational conditions, injuries, and other health concerns
• Epidemiologists, as "population doctors," focus on the total population rather than individual patients
• Epidemiology's goal is to find information to limit undesirable health events and promote positive health factors in a community

Historical Highlights

• Hippocrates (300 BCE) suggested a relationship between disease occurrence and the physical environment
• Benjamin Rush traced yellow fever cases to docks with ships from tropical ports, attributing it to vapors from decaying coffee beans
• Major Walter Reed identified Aedes aegypti as the mosquito carrying the yellow fever virus
• John Snow investigated a cholera outbreak in London in 1849, by tracing the source of the epidemic to water from a communal well, which helped limit disease and deaths
• Robert Koch discovered the organism causing cholera in 1883

Measuring Disease

• Rates enable comparison of health events across different times and places
• Population at risk refers to those susceptible to disease or death from a specific cause
• Incidence rate measures the number of new health-related events or cases of a disease in a population exposed to risk, divided by the total population at risk
• Acute diseases are those in which symptoms peak and subside within days or weeks and usually move quickly through a population
• Attack rate is a special incidence rate calculated for a specific population during a single disease outbreak and expressed as a percentage
• Prevalence rates are calculated by dividing all current cases of a disease (old and new) by the total population
• Chronic diseases last 3 months or longer

Rates and Interpretation

• Crude rates include the total population in the denominator and are useful for comparing similar populations
• Age-adjusted rates are used to show what the level of mortality would be if the age composition of different populations were the same
• Reportable diseases are conditions that health providers, hospitals, or laboratories must report
• Notifiable diseases warrant monitoring, like infectious diseases that can become epidemics

Health Status Measurements

• Mortality statistics are the single most reliable indicator of a population's health status
• Life expectancy is another standard measurement used to compare health status
• Years of Potential Life Lost calculates years lost when death occurs before a standard age. Commonly this age is 65 or 75
• Disability-Adjusted Life Years measure the burden of disease, accounting for premature death and loss of healthy life due to disability

Epidemiological Studies

• Descriptive studies describe the extent of disease in terms of person, time, and place
• Epidemic Curve is a graphic display of disease cases by the time or date of symptom onset
• Common Source Epidemic Curve traces cases to a single exposure source
• Incubation period is the time between exposure to an infectious agent and symptom onset
• Point Source epidemics involve all cases exposed at the same time
• Continuous Source epidemics have cases exposed to a common source over time
• Propagated Epidemic Curve depicts a distribution of cases traceable to multiple sources of exposure

Analytic Studies

• Analytic studies test hypotheses about relationships between health problems and possible risk factors
• Observational studies involve the investigator observing the natural course of events
• Experimental studies involve the investigator allocating exposure or intervention
• It isn't always clear that suspected risk factors cause the development of disease like saying "infection with the malaria parasite causes the disease malaria"

Criteria for Determining Causation

• Strength refers to how strong the association is between the exposure and the disease
• Consistency refers to whether the association has been reported in a variety of people exposed in a variety of settings
• Specificity refers to whether the disease or health problem associated with the exposure is the the only outcome
• Temporality refers to whether the exposure always precedes the disease
• Biological plausibility refers to whether the suspected causation makes sense with existing medical and biological knowledge