Epidemiology of Communicable Diseases

Dynamics of Disease Transmission

• Infection: the entry and development or multiplication of an infectious agent in the body of man or animals, which implies that the body responds in some way to defend itself against the invader
  • Types of infections: colonization, subclinical or inapparent infection, latent infection, and manifest or clinical infection
• Contamination: the presence of an infectious agent on a body surface or on or in clothes, beddings, toys, surgical instruments or dressings, or other inanimate articles or substances including water, milk, and food

Host

• A person or other animal, including birds and arthropods, that affords subsistence or lodgment to an infectious agent under natural conditions
  • Obligate host: the only host, e.g., man in measles and typhoid fever
• Reservoir of infection: a case or carrier, or contaminated food, milk, water, soil, or other substances
  • Types of reservoirs: human, animal, and non-living things (e.g., soil, water)

Case and Carrier

• Case: a person in the population or study group identified as having the particular disease, health disorder, or condition under investigation
  • Primary case: the first case of a communicable disease introduced into the population unit being studied
  • Index case: the first case to come to the attention of the investigator
  • Secondary cases: those developing from contact with primary case
  • Suspect case: an individual who has all of the signs and symptoms of a disease or condition, yet has not been diagnosed as having the disease or had the cause of the symptoms connected to the suspected pathogen
• Carrier: an infected person or animal that harbors a specific infectious agent in the absence of clinical disease and serves as a potential source of infection for others
  • Types of carriers:
    • Incubatory carriers: those who shed the infectious agent during the incubation period of disease
    • Convalescent carriers: those who continue to shed the disease agent during the period of convalescence
    • Healthy carriers: those who emerge from subclinical cases and have developed a carrier state without suffering from overt disease
    • Classification by duration:
      • Temporary carriers: those who shed the infectious agent for short periods of time
      • Chronic carriers: those who excrete the infectious agent for indefinite periods
    • Classification by portal of exit:
      • Urinary carriers
      • Intestinal carriers
      • Respiratory carriers
      • Others

Modes of Transmission

• Direct transmission:
  • Direct contact
  • Droplet infection
  • Contact with soil
  • Inoculation into skin or mucosa
  • Transplacental (vertical)
• Indirect transmission:
  • Vehicle-borne
  • Vector-borne (mechanical or biological)
  • Air-borne (droplet nuclei or dust)
  • Fomite-borne

Susceptible Host

• Four stages of successful parasitism:
  1. Portal of entry: the infectious agent must find a way into the host
  2. Site of election: the infectious agent must reach the appropriate tissue or site in the body of the host
  3. Portal of exit: the disease agent must find a way out of the body
  4. Survival in the external environment: the organism must survive in the external environment until a new host is found

Incubation Period

• The time interval between invasion by an infectious agent and appearance of the first sign or symptom of the disease
• Factors that determine the incubation period: generation time of the particular pathogen, infective dose, portal of entry, and individual susceptibility