Epidemiology

What is Epidemiology and how does it contribute to the health of our society?

A public health science (foundation of public health)

Impacts personal decisions about our lifestyles

Affects government, public health agency and medical organization policy decisions

Where does the term epidemiology come from? Epidemiology derives from epidemic,

a term that provides an immediate clue to its subject matter.

Originates from the Greek words epi (upon) + demos (people) + logy (study of)

Epidemiology

Friis & Sellers 2009, defines epidemiology as concerned with the distribution and determinants of health and diseases, morbidity, injuries, disability, and mortality in populations.

Application of this study to control and prevent health problems in populations.

Historical role of Epidemiology In the past, main causes of death were

due to a single pathogen (disease causing microorganism or related substance)

Epidemiologists had the challenge of isolating a single bacteria, virus, or parasite

Historical role of Epidemiology The discipline of epidemiology underwent

dramatic changes in the first half of the 20th century.

A new kind of epidemiology came into being as a discipline in the decades following the Second World War variously referred to as “modern” or “risk factor” epidemiology (Parascandola, 2011).

Historical role of Epidemiology The new epidemiology differed from

earlier forms of epidemiology in that it included a focus on chronic rather than infectious diseases, an emphasis on identifying individual risk factors for disease, and use of advanced quantitative methodology

(Parascadola, 2011).

What does epidemiology involve?• Study of the health and disease of

the “body politic” – the population.• Basic science of public health

• What causes disease?• How does disease spread?• What prevents disease?• What works in controlling disease?

Major types of Epidemiology Descriptive Analytic

The importance of Descriptive Epidemiology Descriptive epidemiology involves

characterization of the distribution of health-related states or events by: Person – who? Place – where? Time – when? Clinical criteria – what?

Descriptive Epidemiology Describes frequency and patterns of

diseases/conditions Planning, conduction, and evaluation

of effective health education, screening, prevention, and control programs

Analytic Epidemiology

Identifying and quantifying associations

Testing hypotheses, and identifying causes of health-related states or events

Explains why and how health-related states or events occur

Incidence and Prevalence Classic descriptors of how common a

disease, symptom, or problem is in a population

Incidence- measures the occurrence of new disease in a population

Prevalence- measures the existence of current disease in a population

Components of Epidemiology Incidence: Fraction of a group

initially free of the condition that go on to develop it during a given time period. How measured? ▪ By identifying group of susceptible people

(free of disease) and examining them periodically in order to discover and count new cases that develop during interval.

Calculating Incidence Incidence is calculated as the number of new

cases of a disease or condition in a specified time period (usually a year) divided by the size of the population under consideration who are initially disease free.

Example: For example, the incidence of meningitis in the UK

in 1999 could be calculated by finding the number of new meningitis cases registered during 1999 and dividing that number by the population of the UK.

Components of EpidemiologyPrevalence: The fraction

(proportion) of a group processing a clinical condition at a given point in time How measured?▪ By surveying a defined population containing

people with and without the clinical condition at a single point in time (snap-shot in time)

Calculating Prevalence The prevalence is calculated by dividing the

number of persons with the disease or condition at a particular time point by the number of individuals examined. For example:▪ In a study, 6139 individuals completed a

questionnaire (were examined). Of these 6139 people, 519 currently suffered incontinence and so had the condition at the particular time point of the study. Thus the prevalence of incontinence was 519/6139 = 0.085

Prevalence

Prevalence is often expressed as a percentage, calculated by multiplying the ratio by 100.

The above study expresses prevalence as a percentage, thus the prevalence of incontinence is 8.5% (or rounded is 9%)

Components of EpidemiologyFrequency: the number of time an

event occurs Epidemiology studies the number of

times a disease occurs

▪ It answers the question- How many?▪ Epidemiology is a quantitative study

Components of EpidemiologyDistribution: Distribution of an

event by person, place and time Epidemiology studies distribution of

disease

▪ It answers the question: who, where, and when?

Components of Epidemiology

Determinants: Factors or events that are capable of bringing about a change in health(Friis & Sellers, 2009)

Epidemiology studies what determines health events It answers the question : how and why?

Important Epidemiologic terms Endemic: Persistent, usual, expected

health-related state or event in a defined population over a given period of time

Epidemic: outbreak of one specific disease in excess of what would be normally expected (W. Nile)

Pandemic: Epidemic affecting a large number of people, many countries, continents, or regions

Causes of Disease

Identifying causes of disease and the mechanisms by which they spread remains a primary focus of epidemiology

Etiology: Science and study of the causes of disease and their mode of operation

Etiology of Disease

The sum of all factors contributing to the occurrence of the disease

Agent factors + Host factors + Environmental factors = Etiology of Disease

Diseases are classified according to infectivity and communicability

When an infectious disease is contagious, or capable of being communicated or transmitted, it is called a communicable disease

Communicable diseases spread through air, water, food, and contact. Examples:▪ HIV/AIDS▪ Hanta virus▪ Bird flu

Communicable diseases Some — but not all — infectious diseases

spread directly from one person to another. Infectious diseases that spread from person to person are said to be contagious (communicable). Some infections spread to people from an animal

or insect, but are not contagious from another human. ▪ Lyme disease is an example: You can't catch it from

someone you're hanging out with or pass in the street. It comes from the bite of an infected tick.