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•  Pathogens can be species-specific although others can cross species barriers

A pathogen is an agent that causes disease – either a microorganism (bacteria, protist, fungi or parasite), virus or prion

  • A disease is any condition that disturbs the normal functioning of the body (i.e. the body can no longer maintain homeostasis)
  • An illness is a deterioration in the normal state of health of an organism (a disease may cause an illness)

Pathogens are generally species-specific in that their capacity to cause disease (pathogenesis) is limited to a particular species

  • Polio, syphilis, measles and gonorrhoea are examples of diseases caused by pathogens that specifically affect human hosts 

Certain pathogens may cross the species barrier and be able to infect and cause disease in a range of hosts

  • Diseases from animals that can be transmitted to humans are called zoonotic diseases (or zoonoses)
  • Examples of zoonotic diseases include rabies (dogs), certain strains of influenza (e.g. bird flu) and the bubonic plague (rats)



Disease Transmission

Transmission of infectious diseases can occur via a number of distinct mechanisms:

  • Direct contact – the transfer of pathogens via physical association or the exchange of body fluids
  • Contamination – ingestion of pathogens growing on, or in, edible food sources
  • Airborne – certain pathogens can be transferred in the air via coughing and sneezing
  • Vectors – intermediary organisms that transfer pathogens without developing disease symptoms themselves

Methods of Disease Transmission

disease transmission