NURS 236: Principles of Epidemiology

Course Information

• Course title: NURS 236: Human-microbe interactions and epidemiology
• Course part: Principles of epidemiology and disease

Learning Objectives

• Explain the principles of the epidemiology of infectious diseases in humans.
• List Koch's postulates.
• Compare communicable and non-communicable diseases.
• Distinguish terms for the occurrence and severity of disease.
• Recognize the different impacts on the host.

Epidemiology

• Study of disease progression within a population (includes when and where)
• Similar to medical detectives, looking for non-medical factors that contribute to health changes
• Considers etiology, risk factors (including infections, societal practices, control of reservoirs, vectors)
• Early epidemiologists include Snow and Nightingale
  • Snow: Cholera contaminated well
  • Nightingale: Typhus in armies (food, sanitation, disease)

Epidemiology: Types

• Descriptive: Collects all data about the disease being examined (mostly retrospective, after an event—e.g., cholera after the Haitian earthquake)
• Analytical: Analyzes a disease to determine etiology—e.g., Case control & Cohort studies
  • Case control: Compares groups with and without the disease to identify causes or risk factors
  • Cohort: Groups with or without exposure to the disease
• Experimental: Hypothesis testing; how disease spreads, factors influencing spread and treatment; compares control vs. experimental groups, controlling other variables.

Epidemiology: Terms

• Notifiable diseases: Infections reported to public health bodies (http://dsol-smed.phac-aspc.gc.ca/dsol-smed/ndis/list-eng.php—Canada)
• Morbidity: Incidence of specific notifiable diseases
• Mortality: Deaths caused by notifiable diseases
• (NOTE: additional terms will likely be covered later in the course, given the learning objectives)

Koch's Postulates

• Microorganisms are isolated from a diseased/dead animal
• Microorganisms are grown in a pure culture
• Microorganisms are injected into a healthy host
• Disease is reproduced in the host
• Microorganisms are re-isolated from the host

Classifying Human Disease

• Communicable: Spreads between hosts (humans or animals); contagious=easily communicable (e.g., chickenpox, measles, herpes, typhoid fever)

• Non-communicable: Does not spread between hosts; normal microbiota that occasionally causes disease (opportunistic) OR microbes that normally live in the environment causing disease in humans (e.g., Clostridium tetani, Cryptococcus gattii, Naegleria fowleri)

• Classifying Human Disease: Occurrences

  • Incidence: New infections;
  • Prevalence: New and existing infections (more relevant for long-term infections)

• Classifying Human Disease: Frequency

  • Sporadic: Only shows up occasionally (e.g., typhoid fever in the US)
  • Endemic: Always present in an area –E.g. common cold, malaria in certain areas
  • Epidemic: Many new infections in an area over a short period—e.g. seasonal influenza, MRSA in a hospital setting
  • Pandemic: Epidemic disease at a global scale (e.g., AIDS, 1918 influenza)

• Classifying Human Disease: Severity/Duration

  • Acute: Develops quickly and resolves quickly—e.g., influenza
  • Chronic/Persistent: Develops slowly and is longer lasting—e.g., infectious mononucleosis, TB, hepatitis B
  • Latent: Inactive period before producing symptoms—e.g., Shingles

• Classifying Human Disease: Host Involvement/Impact

  • Local infections: Limited to a small area of the body—e.g., boils, pimples
  • Systemic/generalized infections: Throughout the body—e.g., measles; spread by blood or lymph
  • Primary infection: Initial infection
  • Secondary infection: Infection that results from weakening of the immune system by a primary infection
  • Subclinical/inapparent infection: Asymptomatic; carrier of an organism