Investigate modes of transmission of infectious diseases

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Describe a variety of infectious diseases caused by pathogens, including microorganisms, macroorganisms and non-cellular pathogens, and collect primary and secondary-sourced data and information relating to disease transmission, including: (ACSBL097, ACSBL098, ACSBL116, ACSBL117)

  • investigate modes of transmission of infectious diseases, including direct contact, indirect contact and vector transmission

Mode of transmission of infectious diseases:

  • Aerosol:
    • These pathogenic agents are contained in aerosol droplets and are passed from one animal to another, or between animals and humans.
    • These organisms can survive outside the body and remain suspended in the air for long periods of time. They infect others via the upper and lower respiratory tracts.
    • Diseases that are commonly spread by coughing or sneezing include bacterial meningitis, chickenpox, common cold, influenza, mumps, strep throat, tuberculosis, measles, rubella, whooping cough, SARS and leprosy.
  • Direct contact:
    • A susceptible animal becomes exposed through physical contact when the agent from an infected animal or the environment enters open wounds, mucous membranes, or the skin through blood, saliva, nose-to-nose, rubbing, or biting another animal.
    • Some disease agents can spread between animals of different species, as well as to humans.
    • Diseases spread through direct contact are termed as contagious disease and include athlete's foot, impetigo, syphilis (on rare occasions, if an uninfected person touches a chancre), warts, and conjunctivitis.
  • Oral:
    • Consumption of pathogenic agents in contaminated feed, water or licking/chewing on contaminated environmental objects.
    • Food and water contaminated with feces or urine are frequently the cause of oral transmission of disease agents. Contaminated environmental objects could include equipment, feed bunks, water troughs, fencing, salt and mineral blocks, and other items an animal may lick or chew.
    • Includes all forms of herpes viruses, namely Cytomegalovirus infections herpes simplex virus (especially HSV-1) and infectious mononucleosis.
  • Fomite:
    • A contaminated inanimate object transmits a disease agent from one susceptible animal to another and requires a secondary route of transmission (direct contact or oral) for the pathogen to enter the host.
  • Vector-borne:
    • Vectors are organisms that do not cause disease themselves but are responsible for carrying and transmitting infection from one host to another.
    • A mechanical vector picks up an infectious agent on the outside of its body and transmits it in a passive manner. An example of a mechanical vector is a housefly, which lands on cow dung, contaminating its appendages with bacteria from the feces, and then lands on food prior to consumption. The pathogen never enters the body of the fly.
    • Biological vectors harbor pathogens within their bodies and deliver pathogens to new hosts in an active manner, usually a bite. Biological vectors are often responsible for serious blood-borne diseases, such as malaria, viral encephalitis, Chagas disease, Lyme disease and African sleeping sickness. Biological vectors are usually, though not exclusively, arthropods, such as mosquitoes, ticks, fleas and lice. Vectors are often required in the life cycle of a pathogen.
  • Vertical:
    • In this transmission mode, diseases are spread from mother to child (more rarely father to child), often in utero, during childbirth (also referred to as perinatal infection) or during postnatal physical contact between parents and offspring.
    • In mammals, including humans, it occurs also via breast milk (transmammary transmission).
    • Infectious diseases that can be transmitted in this way include: HIV, hepatitis B and syphilis. Many mutualistic organisms are transmitted vertically.

Extract from HSC Biology Stage 6 Syllabus. © 2017 Board of Studies NSW.