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• Factors for the occurrence of an infectious disease

  • Pathogen (virulence of pathogen, mode of entry, and number of organisms)
  • Host (health status, nutritional status, hygiene, age, travel, lifestyle, etc.)
  • Environment (physical factors such as climate, season, and geographic location; availability of appropriate reservoirs; sanitary and housing conditions; and availability of potable water)

  
• Robert Koch


  German physician, considered the founder of modern bacteriology
• Robert Koch


  • Isolated bacillus of tuberculosis
  • Developed an experimental method known as the Koch's postulate that tests whether a particular micro-organism is the cause of a disease
• Koch's postulates


  • A particular microbe must be found in all cases of the disease and must not be present in healthy animals or humans
  • The microbe must be isolated from the diseased animal or human and grown in pure culture in the laboratory
  • The same disease must be produced when microbes from the pure culture are inoculated into healthy susceptible laboratory animals
  • The same microbe must be recovered from the experimentally infected animals and grown again in pure culture
• Once all of the Koch's postulates conditions are fulfilled, it can be concluded that isolated organisms is the cause of the disease
• Limitations of Koch's postulates

  • Koch's postulates involves use of artificial culture media, there are certain organisms that cannot grow in artificial media
  • Not all people that are infected develop clinically-evident disease, most of the time infections are sub-clinical (no apparent clinical manifestation)
  • Difference in genetic characteristics of human beings may modulate their responses to the same organisms (may be mild to some, and may be fatal to some)
  • Certain organisms are species-specific, there are organisms that produce the disease only in humans, thus cannot be tested in healthy susceptible animals
• Stages of infectious diseases

  • Incubation period
  • Prodromal period
  • Period of illness
  • Period of decline
  • Period of convalescence

  
• Incubation period


  The time interval between the entry of pathogen and the appearance of the initial signs and symptoms

  
• Prodromal period


  The period when nonspecific symptoms occur; not all infectious diseases have a prodromal period

  
• Period of illness


  The period when the characteristic features of the illness are present

  
• Outcomes during the period of illness

  • The body's defenses my successfully destroy the invading organisms
  • Failure of the body's defenses to overcome the pathogens leading to fulminant infection
  • Infection may progress from acute form to chronic form
  • May progress into a carrier state – asymptomatic patient but continues to be infective
  • Period of illness may be followed by convalescence, disability or death

  
• Period of decline


  Period when during the number of pathogen particles begins to decrease, and the signs and symptoms of illness begin to decline
• Period of convalescence


  Period of marked recovery, the patient generally returns to normal functions, although some diseases may inflict permanent damage that the body cannot fully repair
• Reasons why infection does not always occur

  • The microbe may land at an anatomic site where it is unable to multiply
  • Antibacterial factors may be present at the site where the pathogen lands
  • Indigenous microbiota of that site may inhibit growth of the foreign microbe (i.e., microbial antagonism)
  • The person may be immune to that particular pathogen
• Types of symbiotic relationships involving microorganisms

  • Neutralism
  • Commensalism
  • Mutualism
  • Parasitism
• Neutralism


  A symbiotic relationship in which neither symbiont is affected by the relationship
• Commensalism


  A symbiotic relationship that is beneficial to one symbiont and of no consequence to the other
• Mutualism


  A symbiotic relationship that is beneficial to both symbionts
• Parasitism


  A symbiotic relationship that is beneficial to one symbiont (the parasite) and detrimental to the other symbiont (the host)
• A change in conditions can cause one type of symbiotic relationship to shift to another type
• Components in the infectious disease process (chain of infection)

  • A pathogen
  • A source of the pathogen (a reservoir)
  • A portal of exit
  • A mode of transmission
  • A portal of entry
  • A susceptible host

  
• Reservoirs of infection


  • Living reservoirs - humans, pets, farm animals, insects, and arachnids (e.g., ticks and mites)
  • Carrier – a person that harbors an infectious agent without discernible clinical disease and serves as a potential source of infection
• Types of carriers


  • Healthy/asymptomatic carriers – infected but are asymptomatic
  • Incubatory carriers – transmit the disease during incubation period
  • Convalescent carriers – recovered from the disease but remain to be capable of transmitting the disease
  • Chronic carriers – harbor the pathogen for months or longer after the initial infection

  
• Zoonotic diseases


  Infectious diseases that humans acquire from animal sources
• Zoonoses may be acquired by direct contact with an animal, inhalation or ingestion of the pathogen, or injection of the pathogen by an arthropod (e.g., rabies, Lyme disease, Dengue)
• Arthropods as reservoirs of infection


  Many different types of arthropods serve as reservoirs of infection, including insects (e.g., fleas, mosquitoes, lice) and arachnids (e.g., mites and ticks)
• Vectors


  When arthropods are involved in the transmission of infectious diseases, they are referred to as vectors
• Examples of arthropod-borne diseases

  • Lyme disease
  • Malaria
• Nonliving reservoirs


  • Air
  • Soil
  • Dust
  • Contaminated water and foods
  • Fomites
• Fomites


  Inanimate objects capable of transmitting pathogens (e.g., bedding, towels, eating and drinking utensils, hospital equipment, telephones, computer keyboards, etc.)
• Communicable disease


  An infectious disease that can be transmitted from one person to another

  
• Contagious disease


  A communicable disease that is easily transmitted from person to person

  
• Zoonotic diseases


  Diseases that humans acquire from animal sources
• Modes of transmission


  • Direct transmission
  • Indirect transmission
• Direct transmission


  Requires close association with the infected host, but not necessarily physical contact
• Forms of direct transmission

  • Skin-to-skin contact
  • Mucous membrane-to-mucous membrane contact by kissing or sexual intercourse
  • Droplet spread: (through coughing/ sneezing)
• Droplets


  >5 microns in size, close proximity (<1m) with the source is necessary for droplets to be transmitted
• Forms of indirect transmission

  • Airborne transmission
  • Vehicle transmission
  • Vector transmission
• Airborne transmission


  Aerosols - <5 microns in size, remain suspended in the air inhaled by susceptible hosts
• Vehicle transmission


  Transmission through contaminated food, water, milk, or biologic substances such as blood or body secretions, fomites