This page is "borrowed" from Dr. John Stevenson at the University of Miami.  

Epidemiology

Definitions

• Epidemiology - study of the occurrence of disease in a human population, especially the cause (etiology) and transmission of disease
• Epidemic - literally, "upon the people" (epi = upon; demos = people); commonly refers to an unusually high incidence of a disease in a community (population) at one time
• Prevalence - the proportion (or percentage) of diseased individuals in a population at one time
• Incidence - the number of diseased individuals in a population at one time

  Did ya know...? The Top 10 Notifiable Infectious Diseases in the US in 1995 were:

  • Chlamydia
    Gonorrhea
    AIDS
    Salmonellosis
    Hepatitis A
    Shigellosis
    Tuberculosis
    Syphilis
    Lyme Disease
    Hepatitis B
• Pandemic - literally, "all the people" (pan = all; demos = people); indicates an epidemic involving more than one continent or a worldwide epidemic
• Endemic - literally, "in the people" (en = in; demos = people); a disease is constantly present, usually at low incidence, in a population
• Outbreak - appearance of several cases of a disease, usually in a short period of time, in an area previously experiencing no cases or only sporadic (scattered or isolated) cases of the disease; "mini" epidemic
• Morbidity - disease; sickness; clinical illness
• Mortality - death
• Reservoir - a site in which infectious agents remain viable (alive) and from which infection of individuals may occur
• Carrier - an infected individual which is not showing obvious signs or symptoms of clinical disease, but which is shedding the etiologic agent for a long period of time (greater than six months)
• Zoonoses - diseases which occur primarily in animals but are occasionally transmitted to people (plague, Lyme disease, rocky mountain spotted fever)
• Vector - living agent which transmits infectious agent (ticks, fleas, flies mosquitoes)
• Fomite - non-living object that transmits infectious agents (pencil, doorknob, handkerchief); fomites such as food or water are called vehicles
• Nosocomial infection - an infection acquired while in a hospital
• Iatrogenic infection - infection caused as a result of medical procedures (catheterization, surgery, injection, etc.) which have a good potential to introduce microbes into patient tissues

Transmission of Infectious Diseases

• Stages in transmission
  • escape from old host
  • travel to new host
  • entry into new host
• Modes of transmission
  • direct - transmission via
    • close, but not intimate, contact (shaking hands, etc.)
    • intimate contact (sexual contact)
  • indirect - transmission via
    • vectors
    • vehicles or fomites

Types of Epidemics

• Common-source
  • infection or intoxication of many people from a single contaminated source
  • characterized by rapid onset, "sharp" peak and rapid decline in incidence
• Propagated
  • introduction of an infected person into a susceptible population leads to transfer of the etiologic agent to others, who transfer it to many others
  • characterized by slow onset, "blunted" peak and slow decline in incidence

Public Health Measures for Control of Epidemics

Involves strategies which break the "chain of transmission"

• Immunization of people
  • boosters frequently required
  • 100% immunization is not necessary (herd immunity)
• Blocking vehicle-mediated transmission
  • water purification
  • effective cooking
  • prevention of food contamination with infectious agents
• Quarantine
  • limiting freedom of movement of individuals carrying infectious agents
  • minimum time required for a quarantine period is equal to longest period of communicability (transmissibility) of the disease
• Elimination of animal reservoirs
  • immunization of animals that act as reservoirs of diseases such as bovine tuberculosis, brucellosis (highly effective when monitored properly)
  • eradication of animals that act as reservoirs of diseases such as tularemia, plague, Lyme disease (sometimes effective, difficult to monitor properly)

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