Epidemiology and Disease Transmission Notes

Epidemiology Overview

• Epidemiology: The discipline studying factors and mechanisms of disease spread in populations (humans, animals, plants).
  • Modern epidemiology initiated by Dr. John Snow during the cholera outbreak in London (1840s).
  • Contributions by Semmelweis in reducing childbirth fever infections in hospitals.

Key Terminology

• Incidence: Number of new cases during a specified time within a population.

  • Example: AIDS incidence in the US (2014) = 37,600 new cases.

• Prevalence: Total number of cases (new + existing) during a specified time within a population.

  • Example: AIDS prevalence in the US (2014) = 1.1 million cases; increased due to longevity of AIDS patients.

• Mortality Rate: Number of deaths in a time frame relative to the population.

  • Example: 1.9 deaths from AIDS per 100,000 people in 2016.

• Morbidity Rate: Number of people affected by a disease.

  • Can refer to incidence rate (new cases per population/time) and prevalence rate (total cases per population/time).

Disease Classification

• Endemic Diseases: Constant presence in a specific geographic area (e.g., Lyme disease in Northeast US).

• Epidemic Diseases: Sudden increase in incidence causing public health problems (e.g., seasonal influenza).

• Pandemic Diseases: Epidemic that spreads worldwide (e.g., 1918 influenza pandemic, COVID-19).

• Sporadic Diseases: Occur unpredictably and infrequently (e.g., Eastern Equine Encephalitis Virus).

Outbreak Types

• Common Source Outbreak: Arises from contamination, with types:

  • Point Source: Temporary source (e.g., contaminated food).
  • Continuous Source: Ongoing source (e.g., ongoing contamination of water).
  • Intermittent Source: Occurs in cycles (e.g., rain contamination).

• Propagated Epidemic: Person-to-person transmission (e.g., common cold).

Disease Transmission

• Types of Transmission:

  1. Contact Transmission: Includes:
  • Direct contact (e.g., rabies, syphilis).
  • Indirect contact via fomites (e.g., doorknobs, toys).
  • Droplet transmission (e.g., colds).
  1. Vehicle Transmission: Via vehicles such as:
  • Water (e.g., cholera).
  • Air (e.g., airborne pathogens).
  • Food (e.g., food poisoning).
  1. Vector Transmission: Through vectors, which are classified as:
  • Biological Vectors: Actively transmit pathogens (e.g., mosquitoes for malaria).
  • Mechanical Vectors: Passively carry pathogens (e.g., flies).

Basic Reproductive Number (R₀)

• R₀: Predicts transmission potential of an infectious disease.
  • If R₀ < 1: Disease will decline.
  • If R₀ > 1: Disease will spread more easily.

Controlling Disease Transmission

• Isolation: Restricting infected individuals to reduce transmission.
• Quarantine: Separating exposed individuals to prevent spread during the incubation period.
• Vaccination: Key to eradicating diseases like smallpox.
• Vector Control: Reducing vector populations (e.g., mosquito control).

Nosocomial Infections (HAIs)

• Nosocomial Infections: Acquired in healthcare settings (1.7 million annually in the US, ~98,000 deaths).
  • Factors include invasive procedures, antibiotic use leading to superbugs, and compromised patient immune systems.
  • Prevention methods: Hand hygiene, proper aseptic techniques, and environmental disinfection.