Establishment of infection

• Contamination


  The presence of an infectious agent on a body surface, inanimate articles or substances including water and food
• Infection


  The entry and development (of many parasites) or multiplication of an infectious agent in the body of persons or animals
• Infectious Disease


  A clinically manifest disease of humans or animals resulting from an infection
• Non-infectious disease


  Diseases not known to be caused by infectious agents
• Communicable diseases

  Illnesses caused by microorganisms and transmitted from an infected person or animal to another person or animal
• Koch was one of the original researchers into tuberculosis, in the 19th century
• In an attempt to define what an infectious disease actually is, koch formulated his famous postulates, which now bears his name
• Koch's Postulates


  1. If an organism can be isolated from a host suffering from the disease

  2. The organism can be grown in pure culture in the laboratory

  3. The organism causes the same disease when introduced into another host

  4. The organism can be re-isolated from that host

  5. Then the organism is the cause of the disease and the disease is an infectious one
• Pathogens


  • Can overcome body defenses and able to effectively colonize the area and establish an infection
• Requirements for a pathogen to be successful in overriding body defense


  1. Maintain a reservoir (a place to live before and after the infection)

  2. Leave its reservoir and enter the host

  3. Adhere to the surface of the host

  4. Invade the body of the host

  5. Evade the body's defenses

  6. Multiply within the body

  7. Leave the body and return to its reservoir or enter a new host
• Common reservoirs for human pathogens

  • Humans
  • Animals
  • Environment
• Incubatory carriers


  Those who can transmit the agent during the incubation period before clinical illness begins
• Chronic carriers

  Intermittently shed the bacteria for a prolonged, ill-defined period of time in the local environment
• Zoonosis


  A human disease that is caused by a pathogen that has an animal reservoir
• New forms of a disease may develop in an animal reservoir and animal reservoirs can make diseases hard to control
• Environmental reservoirs

  Pathogens able to adapt to the differing environments of the human body and environments such as soil and water
• Environmental reservoirs

  • Tetanus (Clostridium tetani, soil)
  • Typhoid Fever (Salmonella typhi, contaminated drinking water)
  • Cholera (Vibrio cholerae, contaminated drinking water)
• Transmission


  The process by which the pathogen leaves its reservoir and enters the host
• Modes of transmission

  • Respiratory droplets (aerosols)
  • Direct body contact
  • Fecal-oral route
• Successful pathogens


  • Able to overcome the body's defenses and attach tightly to cells using adhesins
• Noninvasive pathogens


  Stay where they attach and multiply on these surfaces
• Invasive pathogens


  Penetrate host cells using a class of adhesins called invasins that trigger host cells to engulf them
• Invasiveness


  A survival mechanism that allows a pathogen to enter a nutrient rich environment where it is protected from host defenses
• Evading phagocytosis

  1. Some bacteria produce a slimy extracellular capsule that make it difficult for host cells to engulf them

  2. Others make use of surface proteins that interfere with phagocytosis
• Antigenic variation


  Pathogens periodically change their surface antigens so that the immune system has to constantly manufacture different lines of defense
• Proteases

  Inactivate antibodies (IgA) associated with epithelial surfaces, enabling the pathogen to bind freely to epithelial sites
• Pathogenesis


  How pathogens damage tissue and cause illness
• Bacterial pathogenesis mechanisms

  • Extoxin (secreted enzymes that kill host cells)
  • Endotoxin (part of the outer membrane that stimulate the immune system)
• Exotoxins

  Heat labile, highly toxic, antigenic
• Endotoxin


  Heat stable, weakly toxic, not antigenic
• Viral pathogenesis


  Killing the cells they infect or persisting inside the cells and establishing latent infections that may cause cancer
• Portal of exit


  The anatomical route through which a pathogen leaves the body of the host