epidemiology

• Natural history of disease

  The progression of a disease process in an individual over time, in the absence of human intervention
• Natural history of disease

  • Most diseases have a characteristic natural history
  • Time frame and specific manifestations of disease may vary
  • Influenced by preventive and therapeutic measures
• Natural History of Disease Timeline

  1. Exposure

  2. Pathological changes

  3. Onset of Symptoms
• Exposure


  A susceptible host is exposed to factors sufficient for the disease process to begin
• Pathological changes


  • Occur without the individual being aware of them
  • During this stage the disease is said to be asymptomatic
• Incubation period


  Subclinical stage of an infectious disease
• Latency period


  Subclinical stage for chronic diseases
• Some pathologic changes may be detectable with laboratory or other screening methods
• Intervention at the early pathological stage is likely to be more effective
• Onset of Symptoms


  This phase marks the transition from subclinical to clinical disease
• Spectrum of disease


  The range of disease processes that end either in recovery, disability or death
• Infectivity


  The proportion of exposed persons who become infected
• Pathogenicity


  The proportion of infected individuals who develop clinically apparent disease
• Virulence


  The proportion of clinically apparent cases that are severe or fatal
• Carriers


  Persons with in-apparent or undiagnosed infections who can transmit infection to others
• Types of carriers


  • Asymptomatic/passive
  • Incubatory
  • Convalescent
  • Chronic
• Chain of Infection


  1. Infectious agent leaves reservoir through portal of exit

  2. Conveyed by mode of transmission

  3. Enters through portal of entry to infect susceptible host
• Reservoir


  The habitat in which the agent normally lives, grows, and multiplies
• Types of reservoirs


  • Human
  • Animal
  • Environmental
• Modes of transmission


  • Direct contact
  • Droplet
  • Indirect (airborne, vehicle borne, vector borne)
• Direct contact


  Skin-to-skin contact, kissing, sexual intercourse, contact with infectious soil
• Droplet transmission


  Direct spray over a few feet, before droplets fall to the ground (e.g. sneezing, coughing, talking)
• Airborne transmission


  Infectious agents carried by dust or droplet nuclei suspended in air, may remain suspended and be blown over great distances
• Vehicle-borne transmission


  Transfer of an infectious agent via inanimate objects like food, water, biological products, fomites
• Vector-borne transmission


  Infectious agent carried by animate intermediaries like mosquitoes, fleas, ticks, where the agent undergoes maturation
• Portal of entry


  The manner in which a pathogen enters a susceptible host, must provide access to tissues where it can multiply or a toxin can act
• Host


  Susceptibility depends on genetic, immunity, and other factors affecting ability to resist infection or limit pathogenicity
• Portal of exit


  The path by which a pathogen leaves its host, usually corresponds to the site where the pathogen is localized
• Endemic level


  The amount of a particular disease that is usually present in a community
• Types of disease occurrence

  • Sporadic
  • Hyper-endemic
  • Epidemic
  • Outbreak
  • Cluster
  • Pandemic
• Epidemic


  An increase, often sudden, in the number of cases of a disease above what is normally expected
• Outbreak


  Carries the same definition as epidemic, but for a more limited geographic area
• Epidemic patterns


  • Common-source
  • Propagated
  • Mixed
• Common-source epidemic


  A outbreak in which a group are all exposed to an infectious agent or toxin from the same source
• Propagated outbreak


  Results from transmission from one person to another, e.g. sexually transmitted infections
• Primary prevention


  Prevents the onset of disease, which may be accomplished through the removal of a risk factor. This is achieved through Health Promotion
• Mixed epidemic


  Has features of both common-source and propagated epidemics, a common-source outbreak followed by secondary person-to-person spread
• Secondary prevention


  Detect disease early, treat promptly, cure disease at early stage or control progression of disease. Focused on stage of pre-symptomatic disease. Screening is common - tests detect early physiological indicators of disease
• Tertiary prevention


  The prevention of health deterioration once disease is present. Example: once diagnosed with diabetes, managing insulin levels and regularly examining feet is tertiary prevention, relieving or preventing complications of the disease
• Levels of prevention


  • Control of blood pressure among hypertensives
  • Increasing physical activity
  • Increasing physical activity among diabetics
  • Quitting drinking
  • Eliminating cigarette vending machines from places frequented by teenager
  • Detecting polymorphisms for a breast-cancer gene such as BRCA1