19. Primary and secondary factors of epidemic process (virulence, source of infection, means of transmission, susceptible host)

Basics of infectious disease

• Terminology
  • Colonization = an organism is replicating in a host but causes no disease
  • Sporadic case = case not connected to other cases
  • Outbreak = a greater-than-expected number of cases
    • Endemic = high occurence of a disease within a given geographical area or population
      • Example: Malaria is endemic to sub-Saharan Africa
    • Epidemic = outbreak of a disease within a region in a country, or multiple countries
      • COVID-19 while it was yet to spread out from China
    • Pandemic = global disease outbreak
      • COVID-19 after it had spread to the whole world
• Phases of infectious disease
  • Infection
  • Incubation period
  • Prodromal phase
  • Symptomatic phase
  • Healing/reconvalescence/death
• Levels of infection
  • Colonization
    • E.g. S. aureus in nasopharynx
  • Subclinical infection
    • Before symptoms occur
    • The disease may be contagious at this stage
  • Latent infection
    • After initial symptoms has occured, but before they recur
    • The disease may be contagious at this stage
    • E.g. Latent stage of herpes simplex virus
  • Manifest (clinical) infection
• Seasonality of infections
  • = some diseases have increased number of cases regularly in a certain season
  • Influenza in the winter
  • Diphtheria in the autumn
• Cyclicity of infections
  • = some diseases have increased number of cases regularly at longer intervals
  • Pandemic flu every 30 – 40 years
  • Diphtheria every 12 years

Epidemic process

• Epidemic process = the process of spreading of infectious diseases among people
• Characteristics of infectious diseases
  • Caused by infectious agent
  • Has a well-defined incubation period
  • Has an acute course
  • After resolution the immune system has developed immunity
• Factors of the epidemic process
  • Primary (direct) factors
    • Sources of infection
    • Possible routes of transmission
    • Susceptible human persons
  • Secondary (indirect) factors
    • Natural factors
    • Social factors
• Primary (direct) factors of epidemic process
  • = Those factors that directly influence the spreading of the disease
  • The source of the infection
    • The reservoir is the habitat in which the infectious agent normally lives, grows and multiplies
      • Can be humans, animals or the environment
      • Anthroponosis = an infectious disease that is transmissible from human to human
      • Zoonosis = An infectious disease that is transmissible from vertebrate animals to humans
    • The reservoir may or may not be the source from which an agent is transferred to the host (it may be reservoir -> environment/animal -> human)
      • Source = the location from which the infectious agent is immediately transmitted to the host (not the same as reservoir)
    • Environmental reservoirs
      • Plants, soil, water
      • Many fungal agents have reservoir in soil
      • Legionella pneumophila has reservoir in stale waters
    • Carriers = a person with inapparent infection who can transmit the infectious agent to others
      • Asymptomatic/healthy carrier
        • = carrier who never experience symptoms themselves
      • Incubatory carrier
        • = carrier who can transmit the agent during the incubation period before the clinical symptoms begin
      • Convalescent carrier
        • = carrier who has recovered from the illness but can still transmit the infectious agent
    • Carriage may be
      • Transient
        • = organism can be transmitted only for a short period of time
      • Intermittent
        • = organism can be transmitted sporadically
      • Chronic
        • = organism can be transmitted for a long time
        • Hepatitis B, salmonella typhi
  • The route of transmission
    • Direct transmission
      • Contact
      • Sexually
      • Transplacental
      • Transplantation
      • Respiratory droplets (> 5µm)
    • Indirect transmission
      • Airborne
        • Droplets (< 5µm)
        • May be suspended in air for a long time and can travel great distances
      • Waterborne
        • Through drinking water, bathwater
        • Water can be contaminated with human/animal discharge, sewage
      • Foodborne
        • Food can be infected by improper handling
        • Microorganisms can multiply in food
      • Soil
      • Personal objects
        • Bed
        • Clothes
        • Door handle
      • Insect-borne (vectors)
        • Mosquitoes, fleas, ticks
        • Infectious agent can be carried on the inside of or outside of the insect
  • The susceptible population
    • Susceptibility
      • = a feature of a person that makes it possible for an infectious agent to cause illness in him
    • Individual susceptibility – depends on the immune system of the individual
    • Population susceptibility – depends on the proportion of susceptible and non-susceptible individuals
• Secondary (indirect) factors of epidemic process
  • = Those factors that help the spreading of infections and affect the spread and duration of epidemics
  • Living and working environment
  • Weather
  • Nutritional status
  • Social factors
  • Status of health care

Virulence

• Infectivity
  • = out of 100 people exposed to a certain infectious agent, how many will be infected?
• Pathogenicity
  • = out of 100 people exposed to a certain infectious agent, how many will not only be infected but also sick?
  • Measles, varicella – close to 100%
  • Diphtheria – 30%
  • Poliomyelitis – 1%
• Lethality
  • = out of 100 people who got the disease, how many died?