CHAPTER 14: Principles of Disease and Epidemiology

Definitions and Concepts in Epidemiology

  • Herd Immunity:

    • Defined as the immunity of a significant percentage of a population against a specific infectious disease.

    • Achieved if a high percentage of people in a community is immune (through vaccination or recovery from natural infection).

  • R-naught (R0):

    • A calculated value for communicable diseases representing the average number of people a single infected person can transmit the disease to.

    • Useful for understanding the "spreadability" of an infectious disease.

  • Epidemiology:

    • The study of the occurrence, distribution, and transmission of diseases in populations.

Historical Figures in Epidemiology

  • John Snow (1848-1849):

    • Mapped cholera occurrences in London, significantly contributing to our understanding of its spread.

  • Ignaz Semmelweis (1846-1848):

    • Proven that handwashing considerably decreased puerperal sepsis.

  • Florence Nightingale (1858):

    • Demonstrated that sanitation improvements reduced the incidence of epidemic typhus.

Role of Epidemiologists

  • Determine the etiology (causes) of diseases.

  • Identify important factors and patterns in diseases.

  • Assemble data and graphs to outline disease incidence.

  • Predict the probability of infection spread in populations.

Case Reporting and Outbreaks

  • Enables researchers to establish the chain of disease transmission.

  • Health care workers must report specified diseases to local, state, and national health officials (Notifiable Infection Diseases list).

  • This system provides early warnings for potential outbreaks.

Public Understanding of Epidemiological Data

  • Important for epidemiologists to communicate data in a way that the public can understand.

COVID-19 Data Reporting

  • Weekly COVID Data Update (Example):

    • Daily average cases: 98 (from September 23 to September 29).

    • Weekly positivity rate: 8.9% (from September 26 to October 2).

    • Reported deaths: 1,250.

    • Average hospitalizations: 1,000.

Centers for Disease Control and Prevention (CDC)

  • The CDC collects and analyzes epidemiological data, collaborating with state health authorities to provide necessary health protection information and tools.

  • Regularly releases reports on disease outbreaks and case histories.

Pathology, Infection, and Disease Definitions

  • Pathology:

    • The study of disease.

  • Etiology:

    • The cause of a disease.

  • Pathogenesis:

    • The development process of a disease.

  • Infection:

    • The invasion or colonization of the body by pathogens, which can occur without a detectable disease (asymptomatic infection).

  • Example:

    • Staphylococcus epidermidis is normally on the skin (commensal) but can cause sepsis if found in the bloodstream.

Human Microbiome Overview

  • Acquisition of Microbiome:

    • Begins in utero; lactobacilli acquired during vaginal delivery.

    • Continues through dietary intake and exposure to the environment.

    • Fully established typically by age 3.

  • Composition of Microbiome:

    • Majority of body’s bacteria (90%) located in the gastrointestinal tract; approximately 43 trillion bacterial cells.

    • The diversity and count of these bacteria can vary with lifestyle, diet, vertical transmission, and health conditions.

Predisposing Factors and Disease Patterns

  • Predisposing Factors:

    • Nutrition, sex, genetics, climate, vaccination status, age, lifestyle, and compromised health can increase susceptibility to diseases.

  • Incubation Period:

    • The time between initial infection and appearance of symptoms.

  • Reservoirs of Infection:

    • Humans: carriers with or without symptoms.

    • Animals: zoonoses transmitted from animal hosts (e.g., rabies from bats).

    • Non-living: environments such as soil or water where pathogens can linger.

Disease Transmission Methods

  • Direct Contact: Requires close interaction between host and infected individual.

  • Indirect Contact: Involves fomites (non-living objects that carry pathogens).

  • Droplet Transmission: Spread through airborne droplets, often less than 1 meter.

  • Congenital Transmission: From mother to fetus at birth.

Healthcare-associated Infections (HAIs)

  • Defined as infections acquired while receiving treatment in healthcare facilities, affecting 1 in 31 hospital patients in the US and leading to 70,000 deaths per year.

  • Common sources include contaminated instruments and direct contact with staff or surfaces.

  • Preventative Measures:

    • Handwashing, proper sterilization, and minimizing invasive procedures to reduce risk.

  • Common Types of HAIs:

    • Catheter-associated urinary tract infections, ventilator-associated infections, bloodstream infections, surgical site infections, and gastrointestinal infections.

Summary of Characteristics of Infectious Diseases

  • Communicable vs. Non-communicable:

    • Communicable: spreads from person to person (e.g., COVID-19).

    • Non-communicable: not transmitted between people (e.g., cancer).

  • Epidemic, Pandemic, Sporadic:

    • Epidemic: affects many people in a short time.

    • Pandemic: an epidemic that occurs worldwide.

  • Acute vs. Chronic vs. Latent Diseases:

    • Acute: sudden onset and short duration.

    • Chronic: develops slowly and lasts long.

    • Latent: inactive for a period before causing symptoms.

  • Other Classifications by Severity: includes sepsis (toxic inflammatory condition from microbe spread), toxemia (toxins in the blood), and primary/secondary infections.