11.4-11.6

• Virulence Factors


  • Adhesion
  • Colonisation Factors
  • Invasion Factors
  • Toxins
  • Immune Response Blockers
  • Factors Influencing Disease Transmission
  • Disease Patterns
  • Viruses
  • Viral Structure
  • Diagnosis
  • Prions
• Adhesion


  The component or appendage on the surface of bacteria that helps their adhesion to other cells or to inanimate surfaces
• Colonisation Factors


  Proteins that allow the pathogen to colonise the host’s body
• Helicobacter pylori produces an enzyme (urease) that allows it to survive the acidic conditions of the stomach
• Fungi


  Heat-shock proteins that allow them to grow and reproduce at the body temperatures of mammals
• Some pathogens have a life cycle with different stages, each with different surface molecules which are not recognised by the host immune system
• Invasion Factors


  Proteins that allow pathogens to enter cells
• Toxins


  • Endotoxin
  • Exotoxin
  • Effects
• Endotoxin


  A pathogen toxin bound to its outer membrane and not freely liberated into the surrounding medium
• Exotoxin


  A protein produced and released by a pathogen (e.g., Botulinum toxin)
• Effects of toxins


  • Damage cell membranes
  • Interfere with cell signalling
  • Provoke the immune system to overreact
• Capsule


  Blocks or switches off immune system (e.g., fungi or bacteria)
• Slime Layer (Biofilm)


  Protects from immune system
• Slime layers can collect around instruments or in tissues
• Superantigens


  Bypasses host’s immune response
• Superantigens may still cause potentially destructive inflammatory response
• Factors Influencing Disease Transmission

  • Host
  • Agent
  • Environment
• Host factors


  • Age
  • Sex
  • Genetics
  • Behaviour
  • Nutritional status
  • Health status
• Agent factors


  • Infectivity
  • Pathogenicity
  • Virulence
  • Antigenic Stability
  • Survival
• Environmental factors


  • Weather
  • Housing
  • Geography
  • Occupational setting
  • Air quality
  • Food
• Endemic


  A disease that is constantly present to a greater or lesser degree in organisms of a certain group or living in a particular location
• Epidemic


  The occurrence of more cases of a disease than would be expected in a community or region during a given period
• Pandemic


  An epidemic of infectious disease that has spread across a large region (e.g. multiple continents) or even worldwide
• Virus


  A molecule of protein & nucleic acid (DNA or RNA)
• Viruses are many times smaller than the smallest bacterial cell
• Viruses are non-cellular and must invade living cells in order to reproduce
• Viral diseases


  • HIV/AIDS
  • Chicken pox
  • Hepatitis A & B
  • Influenza
• Genetic Material


  Consists of DNA or RNA
• Viral Shape & Size

  Depends on specific function of virus
• Viruses can have a very small number of genes or up to hundreds depending on the type of virus
• Protein Coat (Capsid)


  Protects the genetic material
• Ways viruses are spread

  • Coughs
  • Sneezes
  • Vomits
  • Bites from infected animals or insects
  • Exposure to infected bodily fluids
• Main types of virus

  • Icosahedral
  • Helical
  • Enveloped
  • Complex
• Icosahedral virus


  The outer shell (capsid) is made from 20 flat sides, which gives a spherical shape
• Helical virus


  The capsid is shaped like a rod
• Enveloped virus


  The capsid is encased in a baggy membrane, which can change shape but often appears spherical
• Complex virus


  The genetic material is coated, but without a capsid
• Diagnosis


  Clinical presentation is used to detect viral diseases
• Symptoms for diagnosis


  • History of severe muscle joint pains
  • Fever
  • Skin rash
  • Lymph gland swelling
• Laboratory investigation is not necessary to detect viral infections, because there is no increase in the white blood cells