Defence Against Disease


  • A disease is a condition that disturbs the normal functioning of the body - in other words, the body can no longer maintain homeostasis
  • An illness is a deterioration in the state of normal health (a disease may cause illness)
  • A pathogen is an infectious agent that causes disease or illness in a host


Stages of Disease

  • Exposure:  Target tissue is exposed to a causative agent
  • Incubation Period:  A latent stage in which no signs or symptoms are apparent (pathogen numbers low)
  • Prodromal Period:  Signs and symptoms are usually mild and non-specific (pathogen numbers building)
  • Clinical Period:  The disease reaches its full intensity and complications may arise
  • Convalescence Period:  A rehabilitation stage when the patient progresses towards recovery after the termination of the disease
  • Recovery Period:  In this stage the patient regains normal health - no signs or symptoms of disease are evident
  • Remission Period:  Some diseases may have a latent phase within the disease progression before a second acute clinical period


Stages of Disease Progression


Transmission of Infectious Disease

  • A host is an organism that has been infected by a pathogen and shows signs of its presence (a carrier is infected but unaffected)
  • Transmission of infectious diseases can occur either through direct physical contact (e.g. body fluid exchanges like kissing, sexual activity, exposure of cut skin) or via indirect means (e.g. air, food or water, vectors, soil, faeces)
  • Vectors are organisms that transmit disease-causing agents from one host to another
  • Some pathogens can only reproduce by spending part of their life cycle in a vector - these are called biological vectors
  • The organism that contracts the adult form of the pathogen is called the primary host and the vector with the juvenile form is an intermediary host
  • Mechanical vectors are not involved in the reproduction of the pathogen and simply carry it from one host to another


Methods of Transmission of Infectious Diseases


Controlling Infectious Disease


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