Epidemiology

Epidemiology Overview

  • Definition: The study of the spread of disease.

  • Focus Areas for Epidemiologists:

    • How does a disease spread?

    • What transfers the disease from one person to another?

    • How to predict the frequency and distribution of a disease in a certain area?

    • Estimation of the number of people affected by a disease.

Key Terms in Epidemiology

Reservoir

  • Definition: The primary habitat of a pathogen, where it replicates and resides.

  • Examples:

    • Humans

    • Animals

    • Insect carriers

    • Water or soil

    • Plants

Source

  • Definition: Where an individual actually obtained the pathogen.

  • Relationship to Reservoir:

    • Often, the reservoir is the same as the source (e.g.,

    • Flu Example: Reservoir = Human; Source = Infected Human).

    • Sometimes can be a fomite (e.g., doorknob, buffet food).

    • Fomite is an inanimate object like a door knob.

Carrier

  • Definition: An individual spreading a disease to others, often unaware of being infected.

  • Asymptomatic Carrier:

    • A person who shows no symptoms but can still transmit the disease (e.g., COVID-19).

    • High risk for spreading the disease unknowingly.

  • Passive Carrier:

    • Transmits pathogens without being infected (e.g., a nurse moving pathogens from one patient to another).

  • Incubation Carrier:

    • Recently infected but not yet symptomatic, can still transmit (e.g., COVID-19 incubation period 6-14 days).

  • Convalescent Carrier:

    • Recovering from a disease but still capable of transmission.

  • Chronic Carrier:

    • Long-term carrier of a disease, continually shedding pathogens (e.g., Hepatitis B or C, Salmonella typhi).

Modes of Transmission

Arthropods/Vectors

  • Definition: An insect or arthropod that carries a disease.

  • Importance: Some diseases can only spread through vectors, not between humans.

Types of Vectors

  • Mechanical Vector:

    • Carries pathogens without a biological change (e.g., flies landing on feces and then on food).

  • Biological Vector:

    • Necessary for the life cycle of the pathogen (e.g., Plasmodium for malaria, requires both mosquito and human).

Zoonotic Diseases

  • Definition: Diseases that spread from animals to humans.

  • Example: Rabies virus, transferable when infected mammals bite a human.

Communicable vs Non-Communicable Diseases

  • Communicable Diseases:

    • Can spread from person to person (e.g., flu, coronavirus, strep throat).

  • Non-Communicable Diseases:

    • Cannot spread human to human (e.g., cancer, heart disease, autoimmune disorders).

    • Some also spread via vectors (e.g., Lyme disease from ticks).

Different Types of Infection/Transmission

  • Direct Contact:

    • Definition: Physical contact or fine aerosol droplets.

      • Including sexual transmission, handshaking.

  • Indirect Contact:

    • Definition: Passes from infected host to intermediate conveyor and then to another host.

      • Vehicle: inanimate material, food, water, biological products, fomites.

        • Involves fomites (e.g., doorknobs, toilet seats) and contaminated materials.

      • Airborne: Droplet nuclei, aerosols.

        • Examples include sharing toys or clothes among children.

Epidemiological Terms

Prevalence

  • Definition: Total number of existing cases in a population, often shown as a percentage.

  • Example: 50% of Connecticut residents currently have the flu.

Incidence

  • Definition: Measurement of new cases over time compared to the general population.

  • Example: 10,000 new flu cases this week.

Mortality Rate

  • Definition: The measure of deaths caused by a disease.

Morbidity Rate

  • Definition: The number of individuals afflicted with a disease at a given current time.

Disease Endemicity and Outbreaks

Endemic Occurrence

  • Definition: Diseases known to occur predictably in certain populations at certain times.

  • Example: Lyme disease in New England during summer.

Sporadic Occurrence

  • Definition: Random occurrence of diseases with no predictable timing or location (e.g., typhoid fever).

Epidemic

  • Definition: An unexpected increase in disease prevalence beyond predicted levels.

  • Outbreak vs Epidemic:

    • Outbreak: Smaller locations or populations.

    • Epidemic: Widespread and not confined to a single location.

Pandemic

  • Definition: An epidemic that is worldwide (e.g., COVID-19, HIV virus).

Nosocomial Infections

  • Definition: Infections acquired in medical facilities such as hospitals and nursing homes.

  • Characteristics:

    • High presence of drug-resistant bacteria, higher pathogen concentrations due to immunocompromised individuals.

  • Pathogen Examples:

    • Staphylococcus aureus (MRSA).

    • Clostridium difficile (associated with antibiotics).

    • Pseudomonas aeruginosa (known for high resistance).