BMSC 5350: Epidemiology Basics

Epidemiology

The study of the distribution and determinants of health-related states or events in specified populations and the application of this study to control of health problems

Etiology

The cause of a disease

Primary Prevention

Action taken to prevent the development of a disease in people who do not have the disease Examples: Vaccines, healthy behaviors, etc.

Secondary Prevention

Identifying people in whom a disease process has already begun but who have not yet developed clinical signs or symptoms of the disease (preclinical phase) Examples: Cancer screening, mammograms, etc.

Tertiary Prevention

Preventing complications in those who have already developed signs and symptoms of an illness and have been diagnosed (those in the clinical phase of illness) Examples: Cancer treatment, physical therapy, etc.

Direct Transmission

Disease is transmitted from person to person by direct contact. Examples: STIs

Indirect Transmission

Disease is transmitted from person to person by a common vehicle like contaminated air or water or a vector. Examples: yellow fever (mosquitos), Lyme Disease (ticks), etc.

Disease

any harmful deviation from the normal structural or functional state of an organism, generally associated with certain signs and symptoms and differing in nature from physical injury. A diseased organism commonly exhibits signs or symptoms indicative of its abnormal state. There is subclinical or non-clinical (change from normal structure or function) vs. clinical disease (change from normal structure or function expressed via signs/symptoms)

Clinical Disease

Characterized by signs and symptoms

Nonclinical (inapparent) disease

Signs and symptoms have not developed. Includes preclinical, subclinical, persistent, and latent disease.

Preclinical disease

Disease is not clinically apparent but is destined to progress to clinical disease

Subclinical disease

Disease is not clinically apparent and is not destined to become clinically apparent. Diagnosed by serologic response or culture of the organism

Persistent (chronic) disease

Disease or infection that can last for many years or even life

Latent disease

Infection with no active multiplication of the agent, only the genetic message is present, not the viable organism

Carrier

Person who harbors the organism but is not infected as measured by serologic studies or shows no evidence of clinical illness

Endemic

The habitual presence of a disease within a given geographic area

Epidemic

The occurrence in a community or region of a group of illnesses of similar nature, clearly in excess of normal expectancy and derived from a common or a propagated source

Pandemic

A worldwide epidemic

Common-vehicle exposure

Cases arise from the same exposure; Example: food poisoning outbreak from bad chicken at a buffet

Herd immunity

The resistance of a group of people to an attack by a disease to which a large proportion of the members of the group are immune

Incubation period

The interval from infection to the time of onset of clinical illness. Often measured as the time from exposure until the onset of clinical disease as it is difficult to determine the exact time of infection.

Epidemic curve

A graph used to characterized the two primary types of outbreaks (common source and propagated). The y-axis is the number of cases and the x-axis is the date of onset each case patient. Note not all epidemics are common source or propagated- for example with some zoonotic or vector borne diseases.

Primary case

A person who acquires the disease from a specific exposure

Secondary case

A person who acquires the disease from exposure to a primary case

Secondary attack rate

The attack rate in susceptible people who were not exposed to the suspected agent who have been exposed to a primary case. Good measure of person-to-person spread of disease

Epidemiologic surveillance

The ongoing systematic collection, analysis, and interpretation of health data essential to the planning, implementation, and evaluation of public health practice closely integrated with the timely dissemination of these data to those who need to know

Passive surveillance

Surveillance in which available data on reportable disease are used, or in which disease reporting is mandated or requested by the government or the local health authority, with the responsibility for the reporting often falling on the health care provider or district health officer. Provider initiated; less resource intensive. Also known as 'passive reporting'

Active surveillance

A system in which staff are specifically contact providers or others to carry out a surveillance program. Often more accurate than passive. Health-department initiated; more resource intensive

Rates

Tell how fast the disease is occurring in the population

Proportions

Tell what fraction of the population is affected

Incidence rate

The number of new cases of a disease that occur during a specified period of time in a population at risk for developing the disease.

Cumulative incidence proportion

Incidence calculated using a period of time during which all of the individuals in the population are considered to be at risk for the outcome.

Person-time

The sum of the units of time that each individual was at risk and was observed. Often expressed in person-months or person-years.

Prevalence

The number of affected persons present in the population at a specific time divided by the number of persons in the population at that time. What proportion of the population is affected by the disease at that time?

Point prevalence

refers to the prevalence measured at a particular point in time. It is the proportion of persons with a particular disease or attribute on a particular date.

Period prevalence

refers to prevalence measured over an interval of time. It is the proportion of persons with a particular disease or attribute at any time during the interval.

Mortality rate

The number of deaths in a population in a certain time period divided by the number of people in that population

Case fatality

What percentage of people who have a certain disease die within a certain time after the disease was diagnosed?

Direct age adjustment

A standard population is used in order to eliminate the effects of any differences in age between two or more populations being compared. Most commonly used method.

Indirect age adjustment

Used when numbers of deaths for each age-specific stratum are not available or in an occupationally exposed population.

Cohort effect

Changes seen are attributable to the characteristics of the particular 'cohort' under study, not the variable being studied.