Infectious diseases

• Free-living


  Microbes living in natural habitat, such as soil and water
• Saprophytes


  Bacteria living on dead and decaying organic matter
• Parasitic


  An organism which lives on a living host and gets nutrition from it, without any benefit to the host
• Commensals


  Live in complete harmony with the host without causing any harm to it
• Pathogen


  Capable of producing disease in host
• Opportunistic pathogen


  Capable of producing disease when the body's immune system is compromised
• Pathogenicity


  Refers to the ability of a microbe to produce disease
• Virulence


  Refers to the degree of pathogenicity of a microbe
• The ability of a microbial species to cause disease is pathogenicity.

  The ability of a strain of a species to produce disease is virulence.
• Cross infection


  Infection transmitted from one person to another
• Nosocomial infection


  Infection acquired in a healthcare setting
• Subclinical infection


  Active infection, but no symptoms
• Infection


  The entry and multiplication of an infectious agent in the body
• Infectious disease


  A clinically manifest disease of humans resulting from an infection
• Classification of Infection


  • Primary infection
  • Re-infection
  • Secondary infection
  • Focal infection
• Endogenous infection


  Infection from inside the body
• Exogenous infection


  Infection from outside the body
• Based on clinical manifestations

  • Asymptomatic
  • Symptomatic
  • Acute
  • Chronic
• Latent infection


  Inactive or dormant, but can be reactivated
• Local infection


  Infection in a specific area of the body
• Systemic infection


  Generalized illness that infects most of the body
• Smallpox has been described in Ancient Egyptian and Chinese writings and may have been responsible for more deaths than all other infectious diseases combined.
• In the 14th century, the bubonic plague killed about 20 million people in Europe alone.
• In the 20th century, the 1918 influenza may have killed up to 50 million people worldwide.
• To date, close to 20 million people have died of HIV/AIDS.
• Infectious agent


  An organism that is capable of producing infection or infectious disease
• Types of infectious agents

  • Bacteria
  • Fungi
  • Parasites
  • Viruses
• Reservoir


  The place for a pathogen to live and grow
• Reservoirs

  • Humans
  • Animals
  • Insects
  • Fomites
  • Blood
  • Body fluids
• Portal of exit


  Refers to the means by which a pathogen exits from a reservoir
• Portals of exit


  • Nose
  • Mouth
  • Mucous membranes
  • Specimen collection
• Modes of transmission


  Refers to how a pathogen can be transferred from one person, object, or animal, to another
• Modes of transmission

  • Droplet
  • Airborne
  • Contact
  • Fecal-oral
  • Vector
  • Vehicle
• Portal of entry


  Refers to the means by which an infection is able to enter a susceptible host
• Portals of entry


  • Nose
  • Mouth
  • Mucous membranes
  • Skin
  • Unsterile equipment
• Susceptible host


  Refers to the person at risk of infection
• Susceptible hosts

  • Patients
  • Elderly
  • Immunocompromised
  • Healthcare workers
• Carriers


  Carry germs, but have no signs or symptoms
• Ways to break the chain of infection

  • Disinfection
  • Hand hygiene
• Ways to break the portal of exit

  • Sealed biohazardous waste containers
  • Sealed specimen containers
  • Hand hygiene
  • Standard precautions