epidemiology

Epidemiology

Basic Concepts of Epidemiology

  • Definition: Epidemiology is the study of how diseases affect the health and illness of populations.

Communicable (Contagious) Diseases

  • Definition: Diseases that are transmitted from one host to another.

  • Examples: Measles, colds, influenza.

  • Transmission Factors:

    • Determined by interactions between environment, pathogen, and host.

    • Control of any of these factors may break the infection cycle:

    • Improved sanitation: Prevents infection.

    • Antimicrobial medications: Kill or inhibit pathogens.

    • Vaccination: Increases host resistance.

Non-communicable Diseases

  • Definition: Diseases that do not spread from host to host.

  • Origin: Microorganisms most often arise from an individual’s normal microbiota or environment.

  • Examples: Legionella pneumophila in water systems of buildings.

Rates of Disease in a Population

  • Epidemiologists' Focus: More concerned with rates than absolute numbers of cases (e.g., comparing a small vs. large city).

Key Metrics

  • Attack Rate:

    • Definition: Percentage of people who become ill in a population after exposure.

    • Reflects factors like infectious dose and immune status of the population.

  • Incidence Rate:

    • Definition: Number of new cases per unit time per population.

    • Measure of the risk of an individual contracting a disease.

  • Prevalence:

    • Definition: Total number of cases at any time or for a specific period in a given population.

    • Reflects the overall impact of disease on society; includes both old and new cases along with the duration of disease.

    • Expressed as cases per 100,000 people.

Morbidity and Mortality

  • Morbidity:

    • Definition: Incidence of disease in a defined population.

    • Example: Contagious diseases like influenza often have a high morbidity rate as infected individuals may transmit to several others.

  • Mortality:

    • Definition: Overall death rate in a population.

  • Case-Fatality Rate:

    • Definition: Percentage of the population that dies from a specific disease.

    • Example: Ebola is feared due to its very high case-fatality rate.

Cases Visualization

  • Graph Data: HIV Prevalence and Incidence in the U.S., 1980-2010.

    • Total Cases Over Time:

    • 1,200,000 cases in peak years.

    • Gradual increase and statistics displayed for the years: 1980, 1985, 1990, 1995, 2000, 2005, 2010.

    • Active HIV/AIDS Infections: Shows trends in cases over three decades.

Reproduction Number

  • Basic Reproduction Number (R0):

    • Definition: Expected number of cases directly generated by one case in a population when everyone is considered susceptible.

    • Characteristics: Not a constant and is dependent on environmental factors.

    • Proportional to:

    • Contact rate.

    • Attack rate.

  • Effective Reproduction Number (R):

    • Takes into account behavioral modifications and the number of recovered and vaccinated individuals in a population.

Disease Classification

  • Endemic Diseases: Diseases that are constantly present in a population.

    • Example: Common cold in the United States.

  • Sporadic Cases: Occasional few cases occurring from time to time.

  • Epidemic: An unusually large number of cases of a disease.

    • Can arise from introduced or endemic diseases.

  • Outbreak: A group of cases occurring at a specific time and in a specific population.

  • Pandemic: A global outbreak of a disease.

    • Example: AIDS.