MODES OF TRANSMISSION

• Direct Transmission:

  • Direct contact
  • Droplet spread
• Indirect Transmission:

  • Airborne
  • Vehicle-borne
  • Vector-borne (mechanical or biologic)
• Herd Immunity - suggests that if a high enough proportion of individuals in a population are resistant to an agent, then those few who are susceptible will be protected by the resistant majority
• Persistent Infections - occur because the body is unable to clear the organism after the initial infection
• Natural History and Spectrum of Disease:

  • Stage of Susceptibility
  • -Exposure
  • Stage of Subclinical Disease (-Pathologic Changes)
  • -Onset of Symptoms
  • Stage of Clinical Disease (-Usual Time of Diagnosis)
  • Stage of Recovery, Disability, or Death
• Infectivity - proportion of exposed persons who become infected
• Pathogenicity - proportion of infected individuals who develop clinically apparent disease
• Virulence - proportion of clinically apparent cases that are severe or fatal